Every 10 waves, the fortress is upgraded to gain another building slot.
I guess I could add a unique feature that each fortress has.
But right now, these are the current traits:
Mountains:
-No land enemies
-No sea enemies
-Limited supply of Water
-Snow
-Rain
-Wind
-Difficult Salvaging
-Lots of Heat
Plains:
-Farther Range
-Sandstorms
-No sea enemies
-Bandits
-Thunder Storms
-Tornadoes
-Limited Water
-High Winds
-Easy Salvaging
-Lots of Heat
-Earthquake
-Sinkhole
Sea:
-No Land Enemies
-Difficult to gain resources
-Whirlpools
-Wind
-Cyclones
-Thunder Storms
-Tsunami
-Tons of water
-Rain
-Difficult to reach