Near the city entry point, or near a critical choke point in your town, find or preferably create a long narrow space confined on two sides by walls, trees, rocks etc. It is best if it is open at both ends. Place a Gatehouse at the back end, so that prefects won't go out, but immigrants will go in to occupy housing. Build a long road though this narrow cul-de-sac or enclave, and on either side, place numerous prefectures, with the minimum necessary supporting housing and engineer posts.

When an invading enemy approaches the front opening of this Cul de Sac, Prefectures will usually flood out the opening to engage them. In such mass, they can be quite formidable. As Pontius Pilatus pointed out, their ability to delay opponents to allow ballista to take aim should not be underestimated, and in vast numbers in a concentrated stream, they they can defeat mighty armies. But, you have to be wealthy.
[Terratheon]

Comments

The cul-de-sac in Terratheon's original post was somewhat deeper.
[Jayhawk]