Everything in a challenge should be at least hinted at, don't add traps without warning: The floor breaks as you walk on it even though it looks like all the other floors.
Add subtle clues as to what things do, want that floor to crumble? Build cracks, use an emitter, give them a clue.
Invisible walls are bad. The last thing anyone wants is to see an obvious jump just to be met half way with a wall that kills or blocks you that you cannot see. If there is an exploit you want to patch up, have the bricks visible and blend in with the scenery: Use a line of built trees in a forest/have a cave-in or rock pile in your cave.
Edit: I should write a guideline for making a good challenge...