It is possible to climb over the wall that surrounds the Zombie Town, using normal game-play! Edit
(note: this was played on alpha version 0.7.0)
You will need to play long enough to accumulate 7 sandbags. You might be able to do with fewer, but this is how I did it...
Once you have the sandbags in your inventory, you will want to start building over at the perimeter wall nearest to Mall Mark. There is a pavement slab up close to the wall, which has a gap. This will allow you to place alternating sandbags and use the small height difference to avoid having to jump the full height of a sandbag (which is possible but I find it very difficult).
When ready to build, make sure you've cleared enough zombies so they will not pester you, and if there are a few at other ends of the map a new wave will not start (assuming you're at a fairly high wave, which you will be by the time you have 7 sandbags).
-walk to the wall where the raised pavement ends.
-stand on the raised pavement
- place a sandbag on the lower pavement, perpendicular to the wall, and as close to the raised section as possible.
-jump from the raised pavement up onto the sandbag.
-place a sandbag, parallel to the first sandbag so that it rests on the raised pavement. This sandbag will end up being slightly higher than the first sandbag.
-walk onto the 2nd sandbag.
-place a 3rd sandbag on top of the 1st sandbag, aligned as closely as you can.
-you should now be able to jump onto the 3rd sandbag fairly easily, as it's not a full-height jump.
- keep placing sandbags in this fashion. You should finally end up standing on the 6th sandbag and placing a 7th sandbag on the other stack (1,3,5,7).
- jump up onto sandbag #7. From here it is possible to jump up onto the main wall.
- you'll see that there is a ledge surrounding the wall, and various platforms extending outward from the ledge.
- make sure your health is full before you jump down off the wall.