Hi, shazam. The Fortress preserves a certain number of the civil population from enemy pillage attack. The number of preserved population for fortress level is available at -
http://www95.imperiaonline.org/imperia/game_v6/game/help/?section=military&lang=en&speed=2#table-military_145. The number of pillages each player is allowed to send is limited up to 4 pillages every 24 hours (recalled pillages count too). Warden general with the skill defender of civilians will increase the number of preserved civilians. Also, deploying defending army in the province can protect the population since it has to be destroyed so that the Pillage could be performed.
In case a province has already been pillaged, it cannot be pillaged for the next 42 hours in realms with speed x4, 17 hours in realms with speed x10 in peaceful times. In a war each province can be pillaged every 21 hours in realms with speed x4 and 8 hours in realms with speed x10.
