Update 3
All types of minions now (technically) exist within the game. Newly purchased minions will now go into the first available space in the dungeon (empty room closest to surface), and when their upkeep cannot be supported the first instance will be removed and all the other minions will move along to take their place. The position of minions is now also shown on a map of the dungeon, which should automatically update with their movement.
I've just started on trying to get invasions to work, the way I'm doing it probably isn't the cleanest or most straightforward method but it seems to be working so far. I've also been putting together a list of upgrades that could be added to a research menu, as well as one of images I'll need for the final version. I'm trying to keep everything as placeholders for now otherwise I'll know I'll use working on images as an excuse for putting off working on the code side of things.
What I'll be working on next is finishing invasion mechanics and fleshing out the goblin tribe at the surface - at the moment you can change numbers assigned to roles but it's purely visual, the different jobs don't actually do anything yet.