Helped out with building 100 buildings.

This one isn't difficult, it just takes time. Houses are easiest, taking only seven logs, seven rocks and a Collectible (which you'll already have twenty-five of laying about from One Tree Hill). Some collectibles are easier than others, though. For instance, the Zen Rock Stack just takes six rocks, while the Eternal Torch takes seventeen magic powders and thirteen rocks. Try to find a cheap one. This achievement is a bore, but it shouldn't be too terrible, and by the end of the game you should have tons of rocks and lumber laying around.