Reduce Poly Count Within Unity? Higher FPS?

My Game Is Running incredibly slow, but when I take out a city, that I have built within it, it runs at 100fps. I can only assume its because of the polygon count, so is there any easy or automated way I can either reduce the polygon count or make the game run a lot faster INSIDE UNITY? Its at around 12fps now, any help would be greatly appreciated.

Reduce the poly count as much as possible.

Secondly Use “occlusion culling”, improves the performance of the game.

Unity - Manual: Occlusion culling.