Yes that's possible to do. It may be tricky to get right, though.

Figure I'd ask here rather than start a new topic as it's kinda similar, but are new building slots possible? It doesn't look like it, but if it is possible, is there a way to regenerate the navmesh to allow walking on the structure? What I had in mind was a 'wall' building that would surround the base with stairs allowing defenders to order archers on top.

The main issues I can see with it though is no functionality for the gates so it would have to be open by default, and also the fact that attacking the 'building' would likely be wonky due to it encompassing the whole base.

I'm guessing it would be best to just implement walls into the actual map rather than being buildable. But if I do that, is there a way to change the meshes to match certain players if I wanted themed wall meshes?

Check the viking 5 mission. There is a wall, that before being destroyed nobody can pass through. Afterwards, everyone can move through it. There are mechanics that allow making gate open/close with proper navigation updates. That's one example, there are others in the game. Some AngloSaxon mission even literally has a gate that opens and you enter it.

Walls can have navigation on top, this is used in several missions. Just make the wall wide enough and make sure that tiles generate on top of it (give it a landscape collision preset).

Placing them on the map is simpler, but if you want to make them buildable you would have to do that similarly to how traps are being built. That might be tricky. If you place them on the map, then it's pretty simple to change them depending on the player nation. At the "GameStarted" event in the level blueprint, just check each player nation and for their starting position swap the meshes.

Awesome, thanks a bunch for this. I clearly need to play more of the campaign and you've given me a great excuse to do so :D