I wrote a post breaking down the idea that the Steel Defender could pull a chariot into combat. Its a little dated but the number crunching still applies. The key thing for this style of combat would be using a delayed attack, however the real benefit is its support role that it could add to a team.
Here are the results of many hours of research on this topic:
TLDR: RAW, Steel Defender takes its turn after yours, it only has Dodge, and you have to Ready Action if you want to attack on its turn after it's moved into melee range. RAI, who am I to plow through the riches of WotC's emersonian mind. Personally I would use simultaneous turns, it's simpler (and 2024 WotC agrees).
I feel like the mounted combat rules are already clunky by nature, but run into a whole heap of other problems with the Steel Defender (SD) rules since we can only use our bonus action on our turn to direct it to take an action on its subsequent turn. Linear time sucks.
Let's first collate some of the conflicting rules:
The SD rules say that the defender "shares your initiative count, but it takes its turn immediately after yours." and "It can move and use its reaction on its own, but the only action it takes on its turn is the Dodge action, unless you take a bonus action on your turn to command it to take another action. "
The Mounted Combat rules say that when you're mounted "you can chose either to control the mount or allow it to act independently. It moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge."
"If you chose controlled, the initiative of the controlled mount changes to match yours when you mount it and it can move and act even on the turn that you mount it."
"If you chose independent, it retains its place in the initiative order."
Now we have to go down another rabbit hole: is SD controlled or independent? At first glance, PHB seems to say we can choose. But it also adds an exception: "Intelligent creatures, such as dragons, act independently." If the benchmark for intelligence is dragon, well the dullest of the lot, a young white dragon, still sits at 6 INT. Luckily, our SD is as dumb as a pile of rocks at 4 INT, so we aren't forced to use it exclusively independently, and should be free to choose. (not that it matters, you will see why)
CONTROLLED + SD RULES
"The initiative of a controlled mount changes to match yours when you mount it."
Reading this, I first assumed that as soon as we mount SD, it's initiative rule gets overwritten by the Controlled Mount initiative Rules. But then I stumbled onto this rule in PHB p.7:
Specific Beats General:"if a specific rule contradicts a general rule, the specific rule wins."
As the SD rules apply only to it, and the mount rules apply to all mounts, the SD rules take precedent.
There is another contradiction between:
SD's "it takes its turn immediately after yours." and "A controlled mount can move and act even on the turn that you mount it."
Once again, the SD rule will take precedent.
However, if we close our eyes and spin around really fast, and we chose for the Controlled Mount rule to override the SD turn rule here, we run into the following issue. Since you can only mount a creature on you turn, does that imply that a controlled mount can move and act simultaneously with us? Check this thread out for a deep-dive on that, but according to Jeremy Crawford the false messiah, no.
We have to stop now and look at what actions SD is capable of now that it's been mounted. According to SD rules, if you haven't used your bonus action to command it, "the only action it takes on its turn is the Dodge action". But the Mounted Combat rules conflict: your controlled mount "moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge."
Once again, if we use the Specific Rule, the SD rule would take precedent, and only the Dodge action would be available by default.
Personally I would leave the action section up to the DM, as imo adding a free Dash and Disengage option for the mount without the player having to use up a bonus action wouldn't be too game-breaking, and it would allow give the player more manoeuvrability as befits a mounted combatant.
I know mixing and matching rules is a cardinal sin, but rules are also meant to be broken by doe eyed DMs, so for the sake of posterity, let's explore what would happen under each interpretation: the mount can move after us (RAW), before us, or take a simultaneous turn.
In this case, I will assume both you and SD are out of melee range attempting to get into melee range so that SD can use its attack. First, you mount SD. You are now stuck and cannot move until SD's turn. The only way for you to use your action to melee attack is to Ready Action an attack that triggers when SD moves in range of an enemy on its turn. The downsides of this is that you cannot use Extra Attack with a Ready Action attack, and it uses up your reaction too (so no AoO, Shield, Mind Sharpener, Radiant Weapon or Repulsion Shield effects till your next turn).
If you're already in melee range, it's less of an issue as you can simply attack, extra attack, bonus action for SD, and then on its turn SD can uses its action, move and reaction. If you don't want to use your BA on SD, it can still Dodge with its action, move and reaction (although don't forget that a mount's dodge only apply to the mount, no the rider).
This RAW method works fine, although every so often you will have to lose an extra attack and a reaction whenever you want to charge at a new enemy from afar. It can also get a bit clunky to have to remember to Ready Action, but it's not too bad since SD's turn is right after ours.
b) SD's turn happens before yours
In this case, SD becomes limited to it's default Dodge, movement and reaction. It can move up to an enemy, Dodge, then on your turn you would attack, extra attack, and bonus action. From what I can see there's technically nothing stopping you from using the bonus action on this round to affect SD on the next round as there is no specified time limit, but that might be considered pushing the envelope a bit.
This method loses out on the lucrative SD attack damage, although it does let you keep your extra attack and reaction you might've lost otherwise.
c) Simultaneous turns
The fun option. This would allow you to interweave your mounts movement and actions with yours. You could charge in on SD, attack, extra attack, bonus action for SD, SD attack/disengage/help/grapple/etc, and then even move back out if your mount has movement left (or dashed). The new 5.5e 2024 rules operate in this way, which means someone at WotC finally tried playing their own game mounted.
INDEPENDANT + SD RULES
"An independent mount retains its place in the initiative order. Bearing a rider puts no restrictions on the actions the mount can take, and it moves and acts as it wishes. It might flee from combat, rush to attack and devour a badly injured foe, or otherwise act against your wishes."
As said before, the SD rules have priority. They should override the independent mount restrictions (which are less specific), as the SD rules states that "it obeys your commands". However, it's hard to tell if this a specific reference to your ability to use your bonus action to command it, or whether you have a general command over it's behaviour. It seems to revert to independence if you are incapacitated, although I assume it would still be under the control of the player when this happens.
If the SD rules take precedent, this method actually works exactly the same as a controlled mount.
Conclusion:
Use the RAW rules if you want to, they are a bit clunky but it's not the worst. You lose out on an extra attack and a reaction due to Ready Action, usually in the first turn of combat, or until you get into melee. Readying an Action could be re-flavoured as you character gearing up for a charge, and if your DM is kind he might allow either the extra attack to function with it or grant you some other benefit, like advantage, extra damage, or even potentially sending the enemy prone like with the Warhorse's Trampling Charge.
My personal favourite is simply using the simultaneous turn rules (as per the new 2024 rules), it's a lot simpler for everyone involved and you lose out on nothing (although abuses of the mounting/unmounting mechanics should be shut down). If I'm feeling cheeky I might even ask the DM for the extra Dash/Disengage that come with all regular mounts' default actions.
They recently changed this in the 2024 rules if your DM is willing to work with you on this. In 2024, Heavy grants disadvantage if your strength (for melee weapons) is less than 13 instead of being dictated by your size.
I am playing a Forest Gnome Artificer and use mainly an infused longbow. I have asked my DM if I could do that and they said that it couldn't work but that is just for me, you can always ask your DM.
I wrote a post breaking down the idea that the Steel Defender could pull a chariot into combat. Its a little dated but the number crunching still applies. The key thing for this style of combat would be using a delayed attack, however the real benefit is its support role that it could add to a team.
Here are the results of many hours of research on this topic:
TLDR: RAW, Steel Defender takes its turn after yours, it only has Dodge, and you have to Ready Action if you want to attack on its turn after it's moved into melee range. RAI, who am I to plow through the riches of WotC's emersonian mind. Personally I would use simultaneous turns, it's simpler (and 2024 WotC agrees).
I feel like the mounted combat rules are already clunky by nature, but run into a whole heap of other problems with the Steel Defender (SD) rules since we can only use our bonus action on our turn to direct it to take an action on its subsequent turn. Linear time sucks.
Let's first collate some of the conflicting rules:
The SD rules say that the defender "shares your initiative count, but it takes its turn immediately after yours." and "It can move and use its reaction on its own, but the only action it takes on its turn is the Dodge action, unless you take a bonus action on your turn to command it to take another action. "
The Mounted Combat rules say that when you're mounted "you can chose either to control the mount or allow it to act independently. It moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge."
"If you chose controlled, the initiative of the controlled mount changes to match yours when you mount it and it can move and act even on the turn that you mount it."
"If you chose independent, it retains its place in the initiative order."
Now we have to go down another rabbit hole: is SD controlled or independent? At first glance, PHB seems to say we can choose. But it also adds an exception: "Intelligent creatures, such as dragons, act independently." If the benchmark for intelligence is dragon, well the dullest of the lot, a young white dragon, still sits at 6 INT. Luckily, our SD is as dumb as a pile of rocks at 4 INT, so we aren't forced to use it exclusively independently, and should be free to choose. (not that it matters, you will see why)
CONTROLLED + SD RULES
"The initiative of a controlled mount changes to match yours when you mount it."
Reading this, I first assumed that as soon as we mount SD, it's initiative rule gets overwritten by the Controlled Mount initiative Rules. But then I stumbled onto this rule in PHB p.7:
Specific Beats General: "if a specific rule contradicts a general rule, the specific rule wins."
As the SD rules apply only to it, and the mount rules apply to all mounts, the SD rules take precedent.
There is another contradiction between:
SD's "it takes its turn immediately after yours." and "A controlled mount can move and act even on the turn that you mount it."
Once again, the SD rule will take precedent.
However, if we close our eyes and spin around really fast, and we chose for the Controlled Mount rule to override the SD turn rule here, we run into the following issue. Since you can only mount a creature on you turn, does that imply that a controlled mount can move and act simultaneously with us? Check this thread out for a deep-dive on that, but according to Jeremy Crawford the false messiah, no.
https://forums.giantitp.com/showthread.php?629883-Is-Treantmonk-right-or-wrong-about-Mounted-Combat&p=25002582#post25002582
1) Steel Defender Actions
We have to stop now and look at what actions SD is capable of now that it's been mounted. According to SD rules, if you haven't used your bonus action to command it, "the only action it takes on its turn is the Dodge action". But the Mounted Combat rules conflict: your controlled mount "moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge."
Once again, if we use the Specific Rule, the SD rule would take precedent, and only the Dodge action would be available by default.
Personally I would leave the action section up to the DM, as imo adding a free Dash and Disengage option for the mount without the player having to use up a bonus action wouldn't be too game-breaking, and it would allow give the player more manoeuvrability as befits a mounted combatant.
I know mixing and matching rules is a cardinal sin, but rules are also meant to be broken by doe eyed DMs, so for the sake of posterity, let's explore what would happen under each interpretation: the mount can move after us (RAW), before us, or take a simultaneous turn.
2) SD's turn has to be before or after ours
If we take Jeremy Crawfords' words as gospel (simultaneous turns aren't a thing) and we also ignore the Specific SD rule on initiative, then we would have to choose one: a mount acts before or after us. The rules don't clarify if you can change this each turn, but our boy JC says it's at the DM's discretion: https://www.sageadvice.eu/rider-on-controlled-mount-wants-to-attack-mid-move-do-rider-and-mount-share-one-turn/
a) SD's turn happens after yours (RAW)
In this case, I will assume both you and SD are out of melee range attempting to get into melee range so that SD can use its attack. First, you mount SD. You are now stuck and cannot move until SD's turn. The only way for you to use your action to melee attack is to Ready Action an attack that triggers when SD moves in range of an enemy on its turn. The downsides of this is that you cannot use Extra Attack with a Ready Action attack, and it uses up your reaction too (so no AoO, Shield, Mind Sharpener, Radiant Weapon or Repulsion Shield effects till your next turn).
If you're already in melee range, it's less of an issue as you can simply attack, extra attack, bonus action for SD, and then on its turn SD can uses its action, move and reaction. If you don't want to use your BA on SD, it can still Dodge with its action, move and reaction (although don't forget that a mount's dodge only apply to the mount, no the rider).
This RAW method works fine, although every so often you will have to lose an extra attack and a reaction whenever you want to charge at a new enemy from afar. It can also get a bit clunky to have to remember to Ready Action, but it's not too bad since SD's turn is right after ours.
b) SD's turn happens before yours
In this case, SD becomes limited to it's default Dodge, movement and reaction. It can move up to an enemy, Dodge, then on your turn you would attack, extra attack, and bonus action. From what I can see there's technically nothing stopping you from using the bonus action on this round to affect SD on the next round as there is no specified time limit, but that might be considered pushing the envelope a bit.
This method loses out on the lucrative SD attack damage, although it does let you keep your extra attack and reaction you might've lost otherwise.
c) Simultaneous turns
The fun option. This would allow you to interweave your mounts movement and actions with yours. You could charge in on SD, attack, extra attack, bonus action for SD, SD attack/disengage/help/grapple/etc, and then even move back out if your mount has movement left (or dashed). The new 5.5e 2024 rules operate in this way, which means someone at WotC finally tried playing their own game mounted.
INDEPENDANT + SD RULES
"An independent mount retains its place in the initiative order. Bearing a rider puts no restrictions on the actions the mount can take, and it moves and acts as it wishes. It might flee from combat, rush to attack and devour a badly injured foe, or otherwise act against your wishes."
As said before, the SD rules have priority. They should override the independent mount restrictions (which are less specific), as the SD rules states that "it obeys your commands". However, it's hard to tell if this a specific reference to your ability to use your bonus action to command it, or whether you have a general command over it's behaviour. It seems to revert to independence if you are incapacitated, although I assume it would still be under the control of the player when this happens.
If the SD rules take precedent, this method actually works exactly the same as a controlled mount.
Conclusion:
Use the RAW rules if you want to, they are a bit clunky but it's not the worst. You lose out on an extra attack and a reaction due to Ready Action, usually in the first turn of combat, or until you get into melee. Readying an Action could be re-flavoured as you character gearing up for a charge, and if your DM is kind he might allow either the extra attack to function with it or grant you some other benefit, like advantage, extra damage, or even potentially sending the enemy prone like with the Warhorse's Trampling Charge.
My personal favourite is simply using the simultaneous turn rules (as per the new 2024 rules), it's a lot simpler for everyone involved and you lose out on nothing (although abuses of the mounting/unmounting mechanics should be shut down). If I'm feeling cheeky I might even ask the DM for the extra Dash/Disengage that come with all regular mounts' default actions.
Let me know if I missed anything!
They recently changed this in the 2024 rules if your DM is willing to work with you on this. In 2024, Heavy grants disadvantage if your strength (for melee weapons) is less than 13 instead of being dictated by your size.
I am playing a Forest Gnome Artificer and use mainly an infused longbow. I have asked my DM if I could do that and they said that it couldn't work but that is just for me, you can always ask your DM.
If you are allowed to use the steel defender as a mount you could use the Saddle of the Cavalier and just be busted.