Thanks for the suggestions! I've been wanting to add the stockpile building for a while, but put it off until I make a new gui.
Enemy settlements will likely have a 'settlement strength' in the full game. This decides how many airships they send your way, and also how likely they are to surrender (which gives you all their buildings). I'm also looking at an ai helper. When stockpiles reach a threshold it will build units a bit like the enemy spawns them. Probably most of this won't happen until near the finished project however.
I'm also thinking of beefing up the airship health, maybe to 6 instead of 2, and reducing enemy spawns. Currently any assaults are extremely costly and end in huge losses, but as we know from real strategy, if you are fighting a fair battle you didn't plan correctly. Of course you can cheese the game by manually controlling units or using seige walls, but I want that to be an advanced tactic rather than the only feasible one.