So I'm a sucker for a tower defence-style game, ever since that little minigame in Final Fantasy VII. I'm also a huge fan of DnD. I know there's a lot of talk in the community along the lines of "oh well if you want to play a different kind of game, you should try a different system instead of modding 5e". I get that and I respect it to a certain extent. On the other hand, my table of players have been on this same campaign for 4 years now. There's a lot of trust here and that gives me the opportunity to experiment.
I've actually run a couple of siege encounters before (because who doesn't love the idea of a group of desperate heroes, outnumbered and outgunned, fending off a far superior enemy force?). I used the mob rules from the Dungeon Masters Guide in the past, with limited success. That system definitely cut down on dice rolls, which was helpful, but I didn't feel like my players had a lot of agency over the actual defences.
Pictured: a staple of my childhood.
So, without further ado, I present my tower defence mechanics for DnD, playtested and approved by my players.
N.B. My party consisted of 4 level 8/9 players when I ran this for them.
The Countdown
First things first, every siege needs an endpoint, even if your players don't know when it's coming. Are they desperately holding out for allied reinforcements (at first light on the fifth day)? Are they waiting for the city's arcane shields to be repaired? Or for the portal to the Abyss to be sealed? Whatever it is, you should have it in mind and know roughly how far off it is. I actively encourage not telling your players. Not knowing how long they have to hold for adds to the desperation and makes it difficult for them to plan.
Rounds
Each defensive round lasts one hour. This means if players want to Short Rest, they're going to miss the full round. Long rests are pretty much out of the question. The idea is to stretch their resources, push them to their limits, and really test that heroism and ingenuity.
Even if the actual fighting in a round is over quite quickly, the players are assumed to spend the rest of that time busy with all the minutae of siege defence: dragging the wounded back from the front line, checking on supplies, getting updates from the other defence zones, etc.
Defensive Zones
This is where your players make their stand. In my game, there were four barricades. In yours, they could be separate walls of a keep, sacred trees that protect a forest, or whatever you need. You could even given them different physical features: maybe one has a vantage point, or some cover, or difficult terrain. The point is, there should be several of them so that defences can be stretched across them. At the start of each Defense Round, I rolled a d4 to see how many barricades would come under attack that round, then rolled again to determine which ones. The players were given this information up front so they could plan the defence accordingly.
Once they're all done planning, the actual combat at each barricade happens just like regular 5e combat. Picture it like arranging the defence from a bird's eye view over the stronghold and then zooming in to the action-packed fight.
My barricades had three "health" levels each. If the barricades weren't actively defended, or the defence was overrun, they lost two levels of health. If they lost all three, they were destroyed beyond repair. If two barricades were destroyed this way, the defence has to fall back to make a final stand and the endgame triggers.
There are other Defensive Zones which are important, but no actual fighting takes place there. In my version, they were as follows:
Command Centre
Field Hospital
Supply Room
Quarters
We'll get to what they do in a bit.
Pictured: your players, probably.
Defenders
I recommend putting at least one named NPC character at each Defensive Zone. Not only does this give players some back-up (whether it's a martial, caster or support NPC is up to you) but also makes them responsible for someone. Crucially, it also gives players who aren't taking part in the actual fighting for that round, something to do - they can control the NPC. More on that in a moment.
NPCs aren't as tough as your heroes, however. They can fight for only two rounds before they have to skip a round to rest. After they've rested, they automatically return to their last position to resume defence. NPCs can spend Hit Dice when they rest.
Defensive Actions
Now the siege wouldn't be fun for very long if it was just fighting for round after round. The aim is to make your players feel like they're in charge of running the defence. I broke the siege down into a number of Defensive Actions to represent this leadership role and responsibility. Some of them require players to go to one of the Defensive Zones, within the besieged stronghold, which results in a Combat Delay. For example, if they go the Command Centre to rearrange the distribution of NPCs, it takes them two full combat turns to get back to the front and rejoin the fight. Their allies will miss them during that time. As resources run thin and HP gets low, they have to be smart about what action they take. This doesn't mean your players are sat around being bored for several rounds due to Combat Delay, though! They can control one of the NPCs, so they're still involved in the fight. (This also gives them an opportunity to play as new kind of character for a short while).
Players can take only one Defensive Action per Defence Round. They are as follows:
Hold - The player mans the defence at a Defensive Zone. Remember, they'll know in advance which ones are being attacked. If a barricade/wall/magic tree under attack isn't manned by at least one player taking the Hold action, the NPCs are automatically overwhelmed and the barricade falls, taking two levels of damage to its health. If the barricade is overrun (i.e. the players there are KO'd in the fight), named NPCs and players have their unconscious bodies dragged back from the line to the Field Hospital (where they can be revived later) and the barricade still takes those two levels of damage. The Hold action is crucial and your players will want someone Holding each attacked zone every round if possible. Combat Delay: 0 rounds.
Repair - I gave my group the ability to restore a single health level to a non-destroyed barricade. They will need to collect the required materials from the Supply Room via the Supply action on a previous round, though. Combat Delay: 1 round (2 rounds if moving to a different Defensive Zone to fight after repairing).
Command - It may be that players want to rearrange the NPCs on the battlefield, especially if the NPCs have different capabilities. From the Command Centre, they can move as many as they like before the attack commences. Combat Delay: 2 rounds.
Supply - A trip to the Supply Room gives players access to various items. I stocked mine with healing potions, a tangler grenage, barricade repairs, flasks of oil, etc. You may wish to include items like Potion of Heroism and Potion of Speed. Just for funsies, I threw in a single wildly powerful Gnomengarde Grenade, which was great fun. A player taking the Supply action takes 1 item from your list. They may use it instantly or keep hold of it. Combat Delay: 3 rounds.
Revive - The Field Hospital is where downed NPCs can be revived. A player taking the Revive action, restores 2d8+8 HP to an NPC. Both the player and the NPC have a Combat Delay of 4 rounds. This is a hefty wait in terms of combat, so it's definitely better to keep your NPCs alive.
Heal - The other use of the Field Hospital is a full heal. I put this on a limit of two uses total. This helps off-set an unlucky player who has a rough round due to some bad dice, giving them an opportunity to come back strong. Combat Delay: 4 rounds.
Rest - This is what the Quarters are for. Players can complete a Short Rest here but it means missing the full Defense Round.
Enemies
My players had no idea what was coming for each zone until they were already there. You might want to drop hints, if you're feeling kind, but I liked keeping it unknown - it kept them on their toes when trying to decide who was defending where.
For the actual enemies, I made up some monster tables and rolled at random. I tried to include a good mix of creatures: some huge siege monsters, flying critters, casters, etc, to keep combat interesting for the party. My aim was to keep them uncertain about what might be coming.
Crisis
Things rarely go to plan in a siege, especially as the pressure increases after hours and hours of relentless attack. To simulate this, I made a Crisis roll at the start of each round after the first, with a rising chance of a Crisis occuring. When a Crisis was triggered, I randomly selected from the below, but you could easily change these or add more. Any trigger Crisis happened after it was revealed where enemies were attacking for the round, but before the actual fighting, allowing players to adjust their plans accordingly.
Morale Crisis - The NPCs are shaken by the battle and refuse to fight this round. One player must take a 3 round Combat Delay to rally them.
Bombardment - One of the Defensive Zones is hit by enemy bombardment and loses 1 health level. NPCs stationed there take 2d10 bludgeoning damage.
Desertion - A randomly selected NPC quits the defence and runs. Remove them from play.
Sabotage - Supplies are stolen from the Supply Room. Randomly remove 1 item from the list.
Surprise Attack - An enemy has managed to get inside the stronghold. The party must fight them off before going back to the defence. (I had a separate monster table for potential Suprise Attack enemies).
Fire - Choose one area at random from the following. A fire breaks out in this area and its Defensice Actions cannot be used this round while the fire is quelled:
Supply Room
Quarters
Field Hospital
Command Centre
There are plenty of potential crises you could add. Maybe a player has to take a 1 round Combat Delay to push some siege ladders back from the wall, or reinforce the gates to the stronghold? Maybe an enemy assassin takes out one of your NPCs, sending them straight to the Field Hospital and leaving the defences short? It's up to you.
N.B. You may wish to exclude Crisis altogether if your players are lower level or you think it might be too much for them.
The Fight
I seperated out the combat by Defensive Zone and intiative, to prevent people getting bored. So, I'd run through initiative for everyone on Zone 1 first, then do Zone 2, etc, then move to the next combat round. The aim was to keep everyone's attention, keep things moving along, and make sure everyone is involved.
NPCs join in the fight too, so add them to the intiative. Players who are on a Combat Delay might wish to take control of an NPC so they can still be involved in the fighting.
At the end of each Defence Round, give your players a chance to breath...but not for long before you start rolling up which zones are going to be attacked next!
Errata
Over the course of the playtest, a lot of questions came up. Can I go to a barriade and spend ten minutes casting Conjure Elemental before a battle? If I'm KO'd can I Short Rest and spend Hit Dice, rather than being revived at the Field Hospital? After the Morale Crisis, can NPCs go to a new barricade, or do they have to go back to their original one? Do creatures attempt to bypass the barricade or do they just fight there? Really, I just went with my gut on all of these. Whatever answer seemed fair and fun was what I went with and I advise you to do the same. The key is to keep up the desperation and the drama, and make it seem like this really could be a last stand.
The Endgame
Speaking of last stands, what happens if two Defensive Zones are lost? A key element of any good siege scene is where the wall falls and the heroes have to fall back to the keep. The stakes have never been higher. This is their last chance at survival. (You might want to trigger the endgame if the players are on their last legs, regardless of how many Zones are left. This could be a narrative event that takes place in your siege.)
At this point, I put my party into a straight combat against an overwhelming foe in which they had to survive only a handful more combat rounds. This was their moment to throw everything at the wall. I encourage leaning heavily into the drama here. Describe NPCs being overwhelmed, walls crumbling, the victorius cries of the enemy and the hate in their eyes.
Then, boom! They've finally made it. Whatever thing they were holding out for comes to pass and, at the eleventh hour, the day is saved because of their stalwart defence in the face of annihilation.
I recommend doleing out a big chunk of XP for surviving this incredibly tough scenario, but also, some kind of tangible in-game reward. A favour from a royal, a boon from a powerful magical being, an increase in fame or status, something that really changes the game for them going forward. They've earned it!
Last Words
These are the rules that worked for me but you're free to modify, reflavour and adjust them however you wish, to suit your game and your party. All I hope is that you enjoy them! And, if you do test them out, I'd love to hear how it goes!