Apart from supporting structures with trees or big rocks, you can build higher by
- raising terrain (can be hidden within a wall
- placing iron gates as pillars (can be placed inside of walls)
- placing iron beams (which are worse than iron gates because they are shorter, i think)

Stone on top of the highest iron structure should be green and wood on top of the hightest stone should be blue. You can place core wood beams on top of the stone to further support your wood structure.