The Twelfth House is the final house of the chart, the house of the unconscious, of solitude, of secrets, and of the dissolution of boundaries. It describes the hidden dimensions of experience: the things you keep secret, the fears you don't acknowledge, the patterns that operate below conscious awareness.
In traditional astrology, the Twelfth House is called the House of Enemies (or Secret Enemies). It describes the hidden enemies (internal and external), the self-undoing tendencies, and the experiences of confinement, isolation, and retreat.
A planet in the Twelfth House operates below the surface of consciousness. The Moon in the Twelfth House gives a deep, private emotional life, and often a need for solitude. Venus in the Twelfth House may indicate secret loves or hidden artistic talents. Mars in the Twelfth House may indicate hidden anger or a tendency toward self-sabotage.
The Twelfth House is naturally associated with Pisces and Jupiter (or Neptune): the sign of dissolution and the planet of transcendence (or illusion). When planets are placed in the Twelfth House, they take on something of this Piscean quality, becoming more sensitive, more imaginative, more porous to the invisible world.