After you complete the Old Sorceress boss fight and leave this zone the monsters will be gone when you come back.
This mansion was the home of Kaim, Sarah, and Lirum in the past. They lived there until shortly after Lirum's supposed death (the fact that non-player characters ask about Lirum in the game suggests that Kaim and Sarah moved away before Lirum's absence was noticed). After the family's vacation of the building, the people of Tosca maintained it until Sarah returned under the guise of the Old Sorceress. Afterward, Kaim and the rest of the party discover it in a wrecked state. In order to preserve her past, Sarah created an alternate world within the mirrors where everything appears to be as they were when Lirum was still a child. The mansion does have ghostly characteristics which hint it may be haunted. At one point you can here a blood-curdling scream and see a person standing in the mirror, however the person quickly disappears.