Ofcourse a 5 meter tall concrete wall would be ideal but good luck building that in the middle of the apocalypse. What do you feel like would be the next best option?
My personal choice currently would be to find rolls of 2 meters tall steel wire fence, locate a secluded hilltop somewhere in the woods near water and pull the fence tight between living trees and finish it with barbed wire on the top, mostly in case of other survivors trying to sneak in.
I'd start by securing a small, maybe 5m x 5m perimeter as the starting point. This can be accomplished in a day and I now have a safe place to sleep in. When I obtain more fencing I would start building a second layer around the first one with about 3 meters of space in between and as my camp grows and I get more supplies I would just keep building additional layers. If there are longer stretches of fence between trees I would add additional posts as well as 45 degree supports to prevent a horde from pushing it over.
Wire fence like this comes with atleast 2 additional benefits of being able to see thru it as well as enabling you to kill zombies thru it with a spear.
You don't even need steel wire. Frontiersmen built them with nothing but an axe, a shovel, and logs.