I love the graphics! Very good pixel art and I liked the UI too
My critique would be that it felt like the outcome of the game is mostly determined by how many mountains the player is given, since they are the only means of removing humans.
Some possible solutions:
- Add effects to the other terrains. Like Forests will only begin being consumed three turns after being played, or only the closest bank of water can be farmed or something
- Slow the spread of humanity. Exponential growth gets out of hand quickly, slowing it may give the player a fighting chance