So, my suggestion in a "user-view" is as follows: right now the development of your town is quite peaceful and easy(ignoring vile mobs at night). IMHO, it would be cool to have random tribes appear that will try to pillage and.... kill you and your people. That would have more impact on the defensive side of the mod. Right now you have no practical need of ballistas or other siege engines - set up a couple of archers and you are fine. But if you know someone might come for you, you will have to think twice about making that farmer instead of, let's say, a warrior.

Now to the (kinda) technical side - each in-game day has a chance of triggering an event(for an individual player, meaning there can be multiple events at once). Once triggered, the corresponding player might (maybe add scouts for that? or make that auto-triggered) receive a message "The neighboring tribes are planning to attack you. Be prepared". After that, Player has, let's say 24 in-game hours(or another randomly generated timer) before the actual event. The timer runs out - a bandit camp/settlement spawns near the player(more difficult - Hostile NPC count depends on the amount of NPCs player owns - 1/3 of your "population" or smth).
After spawning, they will remain within the camp for a while(another timer), giving the player a chance to neutralize the threat. If that is not done, the remeaning NPCs will march towards the Player(or the place with most NPCs, Or the warehouse?).
Should they meet a warehouse(within a certain distance from NPCs), they would steal a certain amount of goods. After killing the Player(or some other trigger, stealing some goods mb), the "Tribe" retreats to the camp and either repeats the raid until it is dealt with(all NPCs or Leader killed) OR they despawn at all.

In conclusion, here is a simplified scheme:
Each in-game day "rolls a trigger dice". If is triggered, player(possibly, maybe add a Scout research or smth) gets informed that he will be attacked soon. A timer starts that is min 24 in-game hours long(to give time to prepare). An encampment spawns. Another timer(24 hours) where the hostile NPCs idle at base. TImer runs out - they attack the specific player OR their settlement(NPCs spawned by this player). Hostile NPCs try to a) kill all NPCs AND/OR b) kill Player OR c) steal loot from warehouse OR d) (least likely realizable imho) destroy as much as possible). After some in-game hours of pillaging, hostile NPCs retreat to their camp and either a) despawn or b) stay there idling(and possibly restarting a raid sometime later).**
Should the Player kill all NPCs(or their leader, this has some downsides because the player can just focus the leader), the event is over and the Player has access to the encampent.

**As an idea - should the player fail to kill the raiders, they could "upgrade" their camp, raising their numbers and what would be needed to clear them(like making it necessary to use siege engines)

That's it. Hope i managed to make it understandable :D