Create a labyrinth using walls as your castle. As the game gradually progresses, expand your labyrinth outwards, creating choke points as you go further. On the walls of the labyrinth, fill it with ranged units preferably around the choke points. *Cough* Flame Throwers *Cough* Build about 20 or more gatehouses with guards for shortcuts in the maze for your own units. Won an award for #1 defense for providing global taunts. :)
It should like like this in the beginning:
At the end it should look something like this:
It is only messy to confuse your opponents. Also not close to the final prototype, but you should have won the game already with this strategy. Use it to crush millions of infantry waves!
It's called "An Awesome A-Maze-Ing Armored Aegis-like Avenue to Hell". For short, AAAAAAH!
P.S.: Trolls so hard the Human Opponent has to click at least twice to get units through a labyrinth with unenclosed keep due to the distance tracking limit. :)