Transparent : tall invisible bars are aesthetically acceptable.
Fragile : make it seem easy and simple but fragile in a way that once broken it becomes hard to climb. If you break something and hurt yourself few people will blame the inanimate object.
Sticky: as soon as you touch it, it secretes superglue. The guards come with an innocuous solvent.
Hidden : fill the moat with a "non-Newtonian" dirt colored fluid. Doesn't look like a moat, but people do fall inside and it's hard to move fast through it.