Hi all, I'm looking for help designing a specific scenario for an upcoming session of my ongoing campaign: putting a stop to a siege caravan.

My party of six 9th-level PCs has just arrived in a besieged city with plans to help the city's defenders, a group of rebels seeking to break away from the rest of the kingdom in order to escape the rule of a despot. The PCs will soon be enlisted to put a stop to the caravan, which is transporting heavy cannons that would almost certainly break the siege if unleashed on the city walls. The only other details I've established about the caravan (based on a scout's report) are the following.

• It is protected by a battalion of elite mercenaries, including at least some cavalry.
• The caravan also includes several warmages, a number of engineers, and some camp followers.
• The caravan is still several days away from the city (and approaching slowly).
• The caravan's leader is a veteran commander who would be prepared for any obvious tactics the PCs might employ.

The PCs can't expect much help from their allies (who will be busy dealing with other threats). Also, simply delaying the caravan would buy time but won't solve the immediate problem, as the besieging army is looking to make an example of the rebels. The rebel leader hopes that a decisive victory (destroying the cannons) would make the siege costly enough for the enemy to consider meeting some of the rebels' demands.

The PC classes and archetypes are Cleric (Forge Domain), Paladin (Oath of Conquest), Bard (College of Whispers), Druid (Circle of the Land), Monk (Open Hand), and Ranger (Beast Master). Note that our house rules have also banned most spells related to flight and teleportation. So with all that in mind:

• How do I design this scenario to make it an interesting challenge for the party?
• What are some ways the PCs might successfully stop the caravan and sabotage the cannons (ideally, destroying them permanently)?
• How does the party deal with such a large group of elite mercenaries?

I have some ideas, but I'm really looking for something to make this scenario more unique, so any off-the-wall ideas would be welcome.

There's a free adventure someone on Reddit made that I think handles this scenario pretty well, though their siege army was more undead-based than yours. Here's a link to the adventure. One of the middle chapters deals with the party riding out from the soon-to-be besieged city to try and soften up the undead army before it can cause too much damage to the town. There are various encounter options and goals that players can meet to help lessen the eventual siege. Using that as a framework, you might be able to come up with some ideas on how best to handle this for your party.

Honestly I would let the pcs figure that out. A lot of talk about sandbox games, and this seems perfect. Give the pcs the lay of the land and as much info as the besieged towns spy network can get. Maybe info on the ebemy commander, is he reckless , or extremely conservative? 

A npc scout to help them find there ideal ambush location would be nice. Maybe,as this seems super important to the besieged town a small unit of troops, enough for a diversion at least. 

Thanks Metamongoose, that sounds great. I'll take a look.

Veth13, I definitely agree. I'm expecting the PCs will probably devise a creative solution to this problem, and the NPC scouts should be able to provide some more information about the caravan. They'll also have an opportunity to do some scouting of their own.

What I'm looking for is some help envisioning some potential solutions (that I can nudge them towards if they're stuck) and on other variables and complications I can throw in to make this a truly unique and interesting challenge for relatively powerful PCs.

Gotcha. You got my mind running if I was a pc what I would do. So here goes.

You have a bard college of the whisper. Any chance there might be a tribe of horse nomads, orcs, elves, or other race that might be convinced to help raid? 

Can the pcs hit the caravan early by sneaking and maybe damaging some of wagons? Makeing the caravan separate into smaller more easier to handle groups.

What is the weather and road conditions. Could the pcs exploit that with magic? Shape stone to destroy a road, or cause massive rain storms to turn the road to mud?

What sorta of creatures live in the area. Could the pcs lead some hungry wyverns, or Griffin from a lair to dine on the enemy horse flesh?

For me, this is all going to come down to terrain. I would start by drawing up a map of the area. Where does the road leave? Can it be blocked, forcing the caravan offroad? Are there ambush points? How would the commander react to those? Is HE familiar with the terrain? If so, how? Scouts? Can they be taken out? Could you kill the scouts, assume their identity, and infiltrate the caravan that way?

Can you infiltrate the camp followers and get at them that way? There's usually plenty of soldiers hanging with the camp followers at night for food or pleasure, and it would be easier to sneak a couple of people in there than into the army proper. Maybe poison the food? I think the camp followers offer you your best "in".

Is there a party of bandits that subsist from the besieged city?  Is it possible that these bandits might be convinced to enter a tenuous pact with the party?  It would be in the party's best interest to stop the caravan from approaching so that the city would be saved.  It would be in the bandits' best interest so as to preserve their primary source of income, looting commerce regarding said city?  A lot of interesting things could come from such an arrangement.  Wow, I am going to have to remember this one for myself.

Cannons mean there is gunpowder. Gunpowder + Fire is not a good combination. 

What terrain is the caravan traveling through? What is the weather?

Destroying the gunpowder isn’t as permanent a solution as destroying the cannons but it should set them back a bit.

I wouldnt create a plan for how the PCs will be able to stop the caravan- That's the PC's job.

Encounter wise, I think you could design this to be interesting by having several different 'sections' of the caravan that players could attack. A wagon could be carrying food, or supplies for the canons, or weaponry, etc. Then if players destroy that section of the caravan, then how does that effect the rest of the caravan? 

Its also worth mentioning that the players might decide to do something interesting depending on what the wagons of the caravan are made of. If I were a player, I might try using torches or spells to light some of the wagons on fire, and then start picking off some of the guards, engineers, etc. during the chaos. So just keep in mind that they may come up with something really creative. 

Thanks all. I was having trouble getting the old gears going, but this definitely gives me a few ideas to work from.