City In the Sky vs. Floating Islands and Cities

The images of the City In the Sky come courtesy of Inhabitat. The idea is to elevate parks, gardens, and perhaps residences high above street level noise, pollution, and crime.

Towers are expensive to build, and once built they cannot be easily moved.

Contrast the sky city with a floating city, which is close to the water, and which can be moved if necessary.

Images of floating islands and cities courtesy of Gizmag:

Dutch Docklands Maldives

Floating islands could be located anywhere from the tropics to the polar regions -- as long as the possibility of encountering ice is accounted for. One could even put a floating island on an alpine lake, as a vacation home away from home.
Always be aware that zombies can travel through the water, and cannot be drowned. If you are looking for shelter from zombies, the tower approach may well be superior to the floating island.