It was supposed to be impossible to find, actually. It was sealed in an eclipse in 1999, supposedly forever, but the seal broke in 2035.

[Just in time for Soma to come of age, to provide Dracula's soul with a young, powerful body....]

The castle is filled with monsters, and the place itself is alive, possibly sentient. It changes layout every time it's summoned. Basically, every inch of it is saturated with chaotic energy, so there are those trying to seal it again before its master awakens.