I believe it's generally accepted that zombies can't climb, or at least, they are really bad at it. So you basically just need to take the high ground. If you can get up a ladder or something, you would basically be safe and could easily pick off any zombies below with a spear or other ranged weapon.

Long term, I would probably go to the mountains, find a clean water source and plant crops. The mountainous terrain would be a big obstacle and most zombies probably wouldn't even try to go up. For any that do, I could probably set up some traps to lead them off an edge where they would fall back to the bottom.