Houses and Mines are your two income generating buildings, they are key to your economy. You should be either population based or mining based economy. Basically, if you're hills or mountains you want to focus on mining and worry less about population (housing and farms). Your military lives in houses and eats food.

Scout towers are essential once you leave protection mode, for best results, keep them at 20%. But, some players find that having less scout towers means more mines and houses and ultimately more military, so there's a balance in there. A lot depends, ask yourself, "do I want it to be easy for somebody to see my military?", if the answer is no, keep them at 20%.

Adamite mines should only be built if you're Goblin or Elf...

Thieves Dens - these are optional, but to me required, it depends, if you really think you won't be stealing very much, then they're not worth it, but, it's nice to be able to steal lumber when you're out, or steal food when you're running low.

Lumber Mills - These produce lumber. You need lumber. Keep these around 3-4%, should be enough for most players.