fun game, I finished the whole thing. mainly the strategy is to build the things that give both gold and people, and then attack attack attack.
on harder levels upgrade a few core buildings that you fully protect, but the same strategy applies and eventually you will overwhelm the enemy. attack using space and then just click their buildings. clicking one building and attacking is too hard.\
only thing is I wish you had like a 'Congrats!' screen or something at the end