Stage 0:
You arrive with a caravan and find a collapsed entrance. NPC miners dig though it while you provide defence from a wildlife and small goblin/kobold groups.
Stage 1:
Miners clear the entrance and you get an access to the first part of the fortress, with a dining room, some bedrooms and early workshops. Miners continue to mine further.
Stage 2:
Miners provide access to the next part of the fortress, with some advanced workshops and a Trade depot. Caravans start to arrive, with useful stuff.
Stage 3:
Miners provide access to a cave river. Now you have a farming plot (with npc farmer) running. I want to implement a requirement to eat, drink and sleep so this should be handy. Also from now on you have to protect the fortress from river attacks from time to time.
Stage 4:
Miners open the next area, with some further achievements.
Stage 5:
Miners provide access to the chasm. From now on you have to protect the fortress from chasm creatures too.
Stage 6:
Miners provide access to the next area, with advanced metalworking and other useful stuff. A dungeon master arrives, with some good and bad results.
Stage 7:
Miners provide access to the magma river. Some nasty critters there
. Also, a wall of adamantine is on another side, so now you can get some cool adamantine gear. A king arrives.
Stage 8: When you mine adamantine for a long time, the HFS is revealed and you're pretty much doomed. Game over.