The Shadowcrest mansion is the Zatara family home. For Zatanna, it is her personal Batcave, a place to store her magical equipment and a place to practice her magical arts. There are books hundreds of years old written in languages that are not human as well as artifacts with the power to do just about anything.
While the building appears to be large on the outside, Mary Marvel notes that it is even larger on the inside.
The outside of the house is littered with wards designed to keep uninvited guests such as enemies or paparazzo away, people who have dared cross its fences uninvited has been reported to suddenly appear a hundred miles away and the ones who have been stupid enough to try and cross violently or using magic have found themselves wading out of the San Francisco Bay having spent the last hour as a mollusk.
Smallville (TV Series)
Shadowcrest was never seen on the show. It was mentioned that Zatanna lives there after the death of her father. It's the family estate of the Zataras'.