These defenses will crumble in a minute, but here`s some suggestions:
-Boiling Oil on gathouses and bastions is a must. You`ll need to research it first though.
-Stone Tipperson on walls is also a very good addition
-Trebuchets on all towers, and make sure you have Flaming projectiles researched for Trebuchets and on during siege
-Get 10-16 regular mobile Trebuchets and put them behind the walls
-Train Oil Cart in the storehouse, and place some outisde of the outer wall. Ignite it with Archers (with fire arrows researches)