Actually there is one thing that stops a player from being able to advance in the maps - special resources like Promethium and Orichalcum. You cannot acquire these any other way than to generate them (other than the few you earn from the maps). I am in Iron Age and I am almost done with level 4 of the Arctic Future map. I do not believe I will be able to go much farther since soon the map will begin to require Promethium for negotiating sectors.