in some levels, when you pass through a space, a wall closes behind you. how do you do that?
It's really an object falling through the ceiling, connected to something above it with push pistons (it should also be possible to use pull pistons from below, but that's less stable). The gap in the ceiling is hidden by tiles in the front layer. The object (usually an anchor) from which the pistons were connected has red/amplitude=distance to move, green/cycle time=2*closing time, blue/duration=0.5. Example screenshot showing hell3's mechanism. The green anchor is what I selected. You can observe its 'color'. Another object, a one-time area switch, is highlighted in red by Gish because that's what the green object is linked to. When there is such a link any pistons connected from the green object won't do anything at first, until the red object is 'activated' (if it's a switch, when it's pressed (or entered in case of an area switch), and if it's a one-time switch it will only activate once). Every time it activates the pistons will move as specified in the green object's 'color'.
so the area swich is located in the object editor, or somewhere else?
The one-time area switch is object type 16, as described in gisheditor.txt.
thx