With the ability to convert enemy forces into friendly units, priests are also a 
very effective (and sneaky) defense against invading armies. Standing behind a good
set of walls, a well-trained priest can easily convert enemy troops and give you 
forces outside your walls. But certain differences between civilizations can give
these units an extra edge. Egypt has a great bonus for priests (+3 range), making
them an excellent wonder-defending race. The Choson are also very good because they
get reduced-cost priests (allowing you to pump out a ton of priests very quickly).