"Like a Battleship, but with much less floating. Which means it’s even less likely to sink!"
Fortresses are a tier 8 Bullets industry. The price is 100 AA Battleships, 100 JJJ Bullets, and 4 Comrades. Fortresses are quite a bit harder to reach and purchase than previous industries. The Battleships cost in particular will require level 2 Mega Tron and Level 5 Megacop (or equivalent - x10 M discount) to feasibly afford.