Mobile Fort

The toughness is the whole pitch: a 0/6 stops nearly everything an aggressive deck of its era could throw at it, and Defender keeps the 0-power body parked behind the front line where it belongs. But the activated ability is the design's actual idea, a pressure valve for the wall archetype's chronic problem: walls win nothing, they only stall. For three generic mana, this one becomes a 3/5 (the +3/-1 shaves a point of toughness off the wall it was) and gets to swing, converting a turn of stalled defense into three points of reach. The once-per-turn clamp is what keeps the maneuver from snowballing within a single turn: without it, you could stack the ability twice for a 6/4 or three times for a 9/3, turning a defensive plug into a burst attacker on the turn of your choosing. As written, the trade is precise and repeatable but never explosive. You spend the mana and the turn's attack to step out from behind your own line, surrendering a point of toughness in the process, which matters because the board you were holding back is still there next turn. Being colorless and built from artifact rather than from a color's defensive toolkit, it sidesteps the usual question walls leave unanswered: how do you actually close. The answer is built in, and it never has to abandon its post for free.