Unfortunately a lot of the rules are not stated (or at least no clearly). But I rant a lot about the rule book quality so I'll get to the point. Page 3 has a diagram showing example moves. Any move shown in red is illegal. Any move that would take the piece off the board is red and is therefore illegal.
Also, there is an planned expansion (siege engines) that talks more about this. You can see the print & plan prototype rules on the official website
. In that expansion there is a rules clarification about being able to flip a tile without moving it (or using an icon's action) - which can only be done if the tile has not icons on the face up side.