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Castle Locthwain gives a black mana base a way to draw cards when the game slows down. It enters untapped if you already control a Swamp, taps for black mana, and can draw a card for three mana and a tap. After drawing, you lose life equal to the number of cards in your hand, making the ability most efficient when your hand is nearly empty.
Mono-black and black-heavy decks can often meet the entry condition with basic Swamps or typed dual lands. Aggressive decks that spend their cards quickly appreciate a land that can help refill after the initial attack stalls. Midrange decks can also use it during an opponent's end step when they have left mana available for interaction but no longer need to cast an answer.
The life payment is determined after the draw, so drawing into an otherwise empty hand normally costs one life. A large hand makes the same activation considerably more expensive. Lifegain from cards such as Gray Merchant of Asphodel can help support repeated use, but the Castle should supplement your draw package rather than replace every other source of cards.
Its Swamp check does not make the Castle itself a Swamp. It lacks that basic land type unless another effect grants it, which matters for fetch lands and Swamp-counting abilities. Include it when the opportunity to draw from a land slot is worth the occasional tapped entry and the mana needed to activate it.
