This started in another thread. The DMG has siege weapons that are really puny. Three level 1 villagers with 10 strength will deal more damage to a castle than a ballista is the same time.

One level 20 fighter without a subclass, with 10str, and a basic greatsword will deal more damage than a trebuchet.

How can we improve them?

Personally, short of any other suggestions I've not heard yet, I'd just give them immunity to things like swords etc. You don't want it to get to ridiculous numbers of dice (do your really want to be rolling 40 dice for each attack of each siege weapon?). It's not like any person short of a Deity could ever do enough with to realistically topple or even significantly damage to a castle. Rather than inflating numbers, just limit what can do damage. For the rare case that there is a crossover, eg Meteor Swarm, perhaps introduce a special resistance that divides the damage by an appropriate amount.

So, in practice, fortified structures like a castle will have a special tag that renders them immune to most attacks, called (say) Fortified Structure. Certain attacks that would logically have an effect on those structures would have a tag, (say) Bombarding. The damage it wouod normally do would be divided by (say) 10 to make it appropriate to the HP of the structures and then applied to buildings with the tag "Fortified Structure". Siege equipment would have the tag (say) Siege, which means that it does full damage against buildings with the tag "Fortified Structure". Attacks or equipment without either the "Bombardment" or "Siege" tags would do zero damage.

Alternatively, we can just leave it to the common sense of the DM. That makes more sense, but if you wanted formalised rules then those would be my suggestions.

Damage Threshold. Big objects such as castle walls often have extra resilience represented by a damage threshold. An object with a damage threshold has immunity to all damage unless it takes an amount of damage from a single attack or effect equal to or greater than its damage threshold, in which case it takes damage as normal. Any damage that fails to meet or exceed the object’s damage threshold is considered superficial and doesn’t reduce the object’s hit points.

That's from the DMG. Figure out what the maximum damage you think shouldn't be able to hurt a castle wall, and set the threshold to that. The strongest hit from a commoner is 24, if you assume they're allowed to get a critical hit using a greatsword against a wall. DT 25 means no ordinary person can harm a castle wall with ordinary weapons.

Edit: I don't think it makes sense to get critical hits on inanimate objects and I'm not sure what the rules say on that. But if you remove it, DT 13 will work just fine, and all the siege weapons will usually surpass that.

There's no hit points for castles because castles don't have "a combination of physical and mental durability, the will to live, and luck." Well, not usually, anyway. So it's impossible to say just how much damage should break down a castle wall.

In the first example, ballista, as far as I knew, weren’t used to attack the walls. It’s a big ol’ crossbow. It was used to fire at the archers on the walls, while staying out of range, to even out the height advantage the defenders had. And if it missed, maybe you got lucky and it fell into the courtyard and hit some rando. The walls were brought down by catapults, trebuchets, or people tunneling under them.

Then in game terms, you have the damage thresholds choiroffire notes, meaning the first 10-20 damage done basically doesn’t count. And that comes off of each hit individually, so the fighter doing 2d6 with no str bonus probably won’t ever hurt the wall, while a trebuchet doing 44 avg damage will take a chunk out most every time. And of course, that level 20 fighter with a greatsword and a 10 str (who has to wear pretty crappy armor with that low of a str) still has to get up next to the wall, while the trebuchet is firing from a long, long distance. Throw in how many, many damage spells can’t target objects, so casters aren’t doing much from range either. And now the siege weapons are quite useful.

Quote from ChoirOfFire >>

Damage Threshold. Big objects such as castle walls often have extra resilience represented by a damage threshold. An object with a damage threshold has immunity to all damage unless it takes an amount of damage from a single attack or effect equal to or greater than its damage threshold, in which case it takes damage as normal. Any damage that fails to meet or exceed the object’s damage threshold is considered superficial and doesn’t reduce the object’s hit points.

That's from the DMG. Figure out what the maximum damage you think shouldn't be able to hurt a castle wall, and set the threshold to that. The strongest hit from a commoner is 24, if you assume they're allowed to get a critical hit using a greatsword against a wall. DT 25 means no ordinary person can harm a castle wall with ordinary weapons.

Edit: I don't think it makes sense to get critical hits on inanimate objects and I'm not sure what the rules say on that. But if you remove it, DT 13 will work just fine, and all the siege weapons will usually surpass that.

There's no hit points for castles because castles don't have "a combination of physical and mental durability, the will to live, and luck." Well, not usually, anyway. So it's impossible to say just how much damage should break down a castle wall.

 Yes, though CoF is still overthinking it a bit. 

This week on "Read all the rules before coming up with poorly informed, unbalanced, and generally half-arsed 'improvements,'" damage thresholds are indeed a thing that exists. In previous editions it was called hardness or damage resistance (different than 5e resistance). If you give that castle wall a DT of 10 and resistance to slashing and piercing those commoners can't damage it at all with personal weapons and a 6d6 catapult will average 11 damage that actually counts. Assuming that a sturdy wall has a few hundred hit points (objects do have hit points, and in such cases HP represent simple physical durability along with damage thresholds) it will take an extended bombardment to break it down this way, which is exactly how catapults worked against well built fortifications historically. It's also why they didn't just have a bunch of guys run up and hit the wall with axes, aside from archers on top of those walls just killing them as they approach. 

Does nobody ever check the Equipment section of the PHB anymore? You've looking for a portable ram.

Edit: Okay, it's for breaking down doors, not walls. Still a useful item.

A portable ram is a device that's meant to be carried by one person.  It's useful for helping a strong person break down a door, it's not going to do anything to a castle wall.

Yeah, that's like what SWAT teams use to breach locked doors. My Barbarian has one. It's nice. Highly recommended. But it isn't siege equipment lol. It *might* get you through a castle gate if you're a superhero, but that's not really what we're here to discuss anyway. There are many ways to enter a castle.

Personally I've always thought about it by tiers of play for martial characters. Level 1-5 you're a talented warrior of some variety go stab that guy, level 5-10 you've surpassed what normal people can do if you wanna cut through a wooden wall and charge the guy on the other side you may succeed depending on what you roll, level 10-15 you're a hero and a mighty warrior you wanna chuck a carriage at someone feel free to roll for it, level 15-20 you're a living legend go kill that castle.

Obviously these are all strength based examples but I feel my point stands, an easier way to do things then via damage threshold is a level requirement.

That works too.

I don't think OP actually has a problem with the damage done by siege weapons. Am I wrong? It sounds like the real issue has something to do with regular weapons. Siege weapons are doing their job just fine on their own, it seems like.

I think that if you're trying to demolish a castle wall with melee attacks, you should have a minimum strength of 21 or a magic weapon or tool specifically enchanted for such a role.

I'm okay with people beating up a castle with a quarterstaff... or a warhammer... or even a sword, or arrows. The PC's should be able to do stupud stuff... like unrealistically break down walls with a dagger.

The DMG gives stats for various materials, like suggested AC and HP. I feel like it's too low, but they want the game to progress, so I have no issue. It's just that when a PC takes a ballista to the face (or a castle for that matter), it shouldn't be less of a deal than getting hit 8 times with a stick.

Siege weapons are useless, unless you’re in a zero magic campaign. Almost any spell of 5th level or higher can easily obliterate any structure.

Quote from VoidFlam3 >>

Siege weapons are useless, unless you’re in a zero magic campaign. Almost any spell of 5th level or higher can easily obliterate any structure.

 Interesting stance. Let's look into a parallel universe. "High level magic is useless. You can't even deal as much damage as a mundane siege weapon."

And then you found yourself on board a ship with an enemy vessel lobbing missiles & projectiles at you while out of the range of your spells…

siege weapons have their place and for every 5th level spell there is the 3rd level Counterspell

”I counterspell the, oh wait… damn”

A wizard is going to run out of spell slots before a catapult runs out of rocks.

That, and carrying around a siege weapon is just funny.

Quote from Erokow32 >>

That, and carrying around a siege weapon is just funny.

 "'Scuse me while I take this trebuchet and enormous wooden horse out of my Bag of Holding..."

Real heroes dual wield them while smoking a cigar.