One, the fact mentioned by others, that you only need to be higher than the surface of a square to be able to create a body and teleport there. The bodies have nonzero height, so you can gain one body-height for free. However, looking at some videos, I'm not sure if that's enough to go to a higher level. Here, at 2:04, it appears that the body created by the player is not tall enough to see the floor of the next-higher level. (That may depend on the version of the game, at least the remake Zenith is different in this.)
But, blocks can be used to go arbitrarily high, limited by energy. When the player creates blocks, they only need to see the side of a block to create a body or more blocks on top.
For example, in this video, at about 0:27, the player creates two blocks and a body, and it appears that the top of the second block is not visible. At 5:20, a similar thing is done, this time without even scrolling the view so that the top of the second block (and the body created there) would be visible. This also shows that they don't need to aim at the top of the block, only at the side. Similarly with three blocks in another video, at about 0:35.
The text on Wikipedia also appears to agree with this:
Controlling Synthoids that are standing at a higher level is fundamental to the game, because only the objects which occupy a visible square can be interacted with (the player may absorb or create objects on a boulder if the sides can be seen).
Height is gained by placing a boulder on any visible square, and putting a Synthoid on the boulder. The player may then transfer consciousness to the new Synthoid, and absorb the old one. Stacks of boulders of any height may be created, if the player has enough energy.
The energy required for creating blocks/boulders is of course acquired by absorbing trees (and sentries!) from the landscape, and reabsorbing blocks and bodies created by the player.