Azkaban Fortress.

Azkaban is a wizard's prison located on an island in the middle of the North Sea. Presumably, it only serves the magical community of Great Britain, as there are other wizarding prisons elsewhere such as Nurmengard. Using certain Charms, the prison is hidden from the muggle world, and is unplottable. It is thought to have undetectable extension charms placed on it (or similar types of magic) to make it bigger on the inside, as the prison seems to serve the whole of wizarding Britain.