[HoMM V]9 Race - Swamp! Is it needed?

I don't know if everything related to buildings has been considered, but I'll write this: it should be possible to create a building called "wasp nests" or "hives," which, during a castle siege, would cast the "wasp swarm" spell on the attacking forces, using a formula like this: magical power x city level x number of days (of course, there should be a limit on the number of days, such as dividing it by 4 or 3...).