If a priest use fort on the commander (Horde - Overlord Agmar and Alliance - High Commander Halford Wyrmbane) in the fortress, they get additional 30% hp or 300k hp (from 800k to 1111k).
I think this is an exploit because the Fort do not generally increase HP by that much.
However Kings will not do anything to the commander.
This is very useful when both fractions got the gate down.