1) Go to Morvayn's Peacemakers house.
2) When facing the entrance, look to your right. You should see a rock by the gate.
3) Jump on the rock, then face the part of the house with the blue roof.
4) Jump to the blue roof part of the house, move to your right and jump to the actual part of the roof.
5) Get to the end of the roof, and jump through the branches to the next house's roof.
6) Do the same thing again
7) Do the same thing again. Now, you should be at a house with a chimney very close to the city walls.
8) Jump on top of the chimney, and jump onto the walls.

If you did this right, you should have successfully jumped into the walls of Anvil.