I remember reading a choose your own adventure style book when i was a kid, and it had an interesting trap that would fit in well in a wizards tower.
It was a elaborate pool. filled with what looked like clear water. In the center of the pool, under the water an ornate key can be seen, most likely the key that opens the door to the next room.
Upon reaching into the water for the key you grasp nothing, until the pool itself springs to life and attacks!!!
Basically the pool is home to a water elemental, which is invisible in the pool until someone gets close enough to be attacked... The mystery of the key is much simpler, and not at all magical. In the still pool an image of the key can be seen, but upon disturbance the reflection dissapates... thats right, a reflection. The key itself hangs from the ceiling, above the line of sight of the normal traveller. It'd be a good trick for those rival wizards, who would be trying in vain to find out what magics are protecting the key.
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