Just wait until you find the Zombie Survival Guide(its a real book, which Ive been trying to locate...though my friend wont sell me his), then the ideas will be pouring in. And as for the safest current "fortress" I can think of...Alcatraz. One way in, just set something so that if zombies start walking under water you can easily stop them(some sort of auto reseting, non-explosive/ammo using device). Personally, I would set the entire perimeter(except for a small path which is HEAVILY guarded by 10foot walls with murder holes set at about average head height) with a trap that has master made Katanas at the right level to shop off their heads, or something to cut them in half when they get out of the water. Even if it only slows them, an industrial strength steel I beam wall would be set up 15feet from shore so that there is something else to protect in the case of amphibious assault.