[Psitticine]

Any army in the province who isn't hiding or sneaking will get involved in any battle that occurs there. The only exception is the Defend Castle order, which causes people to stay in the castle instead of meeting aggressors when they enter the province.

This, BTW, is the reason for the Move and Patrol order. If it were just Move, units would go to the new province (all movement between friendly provinces occurs before movement into enemy provinces) and then go to Defend by default. If an enemy attacks, they'd go into the castle instead of fighting. Move & Patrol tells them to switch to Patrol instead of Defend after the move, allowing them to engage in any battles that occur during the same hosting.