Ice Floe

A defensive land that pays for its protection in the rarest currency a permanent has: its own readiness. The trick lives in the locking clause. Tapping it freezes a ground attacker, and that creature stays frozen only as long as Ice Floe itself stays tapped, so the card grants explicit permission to keep it tapped through your untap step. You are not spending mana (it makes none); you are spending the land's availability, surrendering one untapped permanent every turn for an indefinite hold on a single attacker. That is a slow garrote rather than a momentary parry. The targeting is deliberately narrow: only creatures attacking you, and only those without flying, which makes it a wall against the grinding ground assault rather than a general-purpose tapper. That restriction is what keeps a zero-cost land from being a free Pacifism on a stick; the cost is paid in tempo, one held-down land at a time, for the duration of the lock. The design comes from a school of manabase building where lands carried combat utility that later sets pushed onto spells and creatures: this one does combat math instead of making mana. The asymmetry in the clause (you decline to untap; the creature's controller has no such choice) is the whole engine, and it rewards pinning one threat indefinitely while the board around it sorts itself out.