City Story Metro Hints for iPhone/iPad
When you have removed all of the trees make sure buildings are attached to a road otherwise they will not function.
Tap on your 'Social' menu and start visiting neighbors when you are out of energy but close to leveling up. Doing this will earn you XP points to level up and if you have a lot of neighbors your energy meter will be replenished as a reward.
Before you purchase a building check to see how big it is as you may need to expand before you can place another building.