protip:
if you have all tiles at least 7, the best use of your resources is to put one tile at a time up to lvl 10. say you put a single wood tile to lvl 10, it costs whatever, 40k resources from 7-10. then you can build the sawmill which is wicked cheap for the first 3 levels, giving you a 15% boost to total wood production. then do the same thing for clay and iron. meanwhile, leave all your other tiles at lvl 7 until you've accomplished this. then you can start raising the others.