How do I get down from the floating ledge in the arch room in Fez?

In the full game, there is a treasure map (you can look at both side of the map by rotating it the same way you change perspective during the game) explaining how to get there.

There is no ledge, even in the full game, only invisible platforms.

Not sure if the trial version has treasure maps though.

A treasure map shows the hidden platforms on the "broken arch" level. The map is found in the lighthouse room, but you don't need it to use its secrets.

You need to rotate the view in midair to reach the second hidden platforms.

Alternative method:

In New Game+, Gomez can fly (press ↑↑↑↑ and jump), which is an easy way to descend from platforms.