House 12 represents the most inward dimensions of experience: introspection, withdrawal, imagination, spiritual life depending on the approach, unconscious processes.
It is not a "negative" house despite its reputation in some ancient texts: it is rather a space of depth and stepping back, just before the cycle of houses begins again at House 1.
Cadent house (with Houses 3, 6, and 9): the last of the cadent houses, associated in classical doctrine with adaptation — House 12 comes after the angle of the Ascendant (House 1, counting from the preceding angle in the order of the zodiac) and closes the cycle with a period of withdrawal before it begins again.
Natural sign and ruler: Pisces, ruler Jupiter (Neptune in modern astrology) — consistent with the theme: Pisces is itself the sign most associated with imagination and the dissolving of boundaries. See the Pisces entry.
Planetary joy: Saturn finds its joy in House 12 (Hellenistic doctrine) — the solitude and hardship associated with this house are considered, within this specific framework, favorable ground for this planet rather than hostile.