The Cliff Key is puzzle #14 in Virtual Villagers 3: The Secret City.
First, complete puzzle #3 (The Alchemy Lab). Then, get the following plants: Rose + Cactus + Lotus. Have a villager capable of working (not a child or nursing mother) drink the potion, and he/she will feel "Light and Springy". Bring him/her near the cliff with a pink diamond on top, located near the rose plant at the west of the village. Then, your villager will finally grab the key and bring it to the door, completing the puzzle.