A very effective way of keeping enemies out of your city
is the deathtrap. To build a deathtrap, find a suitable
place to build a wall to keep out your enemies. Now, we
all know that a wall almost never keeps out enemy forces,
sooo build another wall about 10 spaces behind it. Make
sure these walls have at least 5 towers each in order 
for this to work.

Here is what should happen:
Your enemies come charging along and run into your 
first wall; no problem. Your towers pick off about 
2 guys before your enemies break through a small 
segment of your wall. As they flood into the space
between the two walls, the towers on both sides will
shoot at them and they will be destroyed very quickly.
Since your enemies won't bother to take down your 
towers when they break through, this takes very 
little maintenance. It is also very fun to watch. 
For added effect station a legion in the middle and 
see what happens.

by Pete