Hans writes, "I have now completed The Horde without once building a wall or fence and on many levels without even having to swing once!!! The trick is to use your cows as a shield. Place in a wall around your town in patterns like this:

Cow
Cow Mercenary Cow
Cow

The cows attract the Hordlings which are then clobbered by the Mercenary in center. This does not make the game a no brainer though. Watch out for the following:

Cows do not produce gold when they are grazing on rocky land. The cows will eventually eat right down to the rocks and you will have to move them (again and again and again).
On the higher levels the Mercenaries can only effectively protect two or three cows at a time. Clump the cows together for additional cross coverage.

Be careful if you use the dragon. He will roast your cows. ALWAYS, ALWAYS, ALWAYS, pick up all (or most) of your cows and mercs at the end of a level (ie. winter). Otherwise, you are throwing lots of money away and you won't have enough to buy a good herd of cows in the next region.