This was really fun! Its simple but does a lot with it. Do the colour of the population do anything specific? I saw blue, purple, red and wasnt sure if they related to anything. Reading chatnoir's comment I had the same thought, at first I thought I had to use the fire on the trees or something - took me a second to comprehend the squares with water. But I think it'd be cool to add player influenced placement.
Great job on the game!!