It used to be that a building and its staffers were immune to conversion. Now, however, some priests can utilize something known as localized interdict, i.e. a threat of excommunication. This may convince the building owners to switch sides. Well, it at least allows a priest to try.
The technology has a similiar effect on siege equipment and Bohemian war wagons, as it enables the researcher's clergy to have a crack at converting them. However, monks must approach trebuchets, rams, and buildings and speak to the staff (or operators) at close range.
Castles, town halls, monuments and churches are unable to be converted. Farms and fish traps also cannot be converted, though they may be taken over by peasants and fishing ships, respectively, if abandoned. Guild Halls and towers can also be converted, but due to a priest's lack of armor such operations can be deadly and should not be undertaken without significant numbers of priests (i.e. at least 10 or 15).
Researched at: Church
Cost: 475 Florins