Moat

The original prison lock piece, and the card that taught Magic what a true ground-stop costs. The design is brutally simple: a flat denial of the attack step for anything earthbound, with no triggered cost, no upkeep, no decay. White had printed walls and damage prevention before, but this was the first enchantment to address combat as a category rather than as a series of individual problems. The asymmetry runs deeper than it first reads: this grounds everything without flying, so reach, shadow, and unblockable threats are stopped as cold as a vanilla bear. Only true evasion in the air gets through. Build a deck around fliers and the card reads as "your opponent cannot win the game through combat." That open-ended denial is why it has stayed off most reprint sets: the Reserved List status is the headline reason it has not returned, but even setting that aside, Wizards spent the years after Legends carefully unwinding the lock-piece philosophy it established. Every successor (Magus of the Moat, Ensnaring Bridge, Solitary Confinement, Glacial Chasm) takes a piece of the effect and pays a real cost for it: a body that dies, a hand-size clause, an upkeep tax, a cumulative one. Moat pays nothing after the four mana. It is the benchmark against which every prison-white card since has been priced, and the reason the modern color pie treats unconditional combat denial as a tax-bearing effect rather than a printable one.