Steel Wall

Four toughness for a single generic mana, no color requirement, no upkeep cost: this is about as cheap as a durable blocker gets, and that universality is the whole pitch. Artifact creatures had been a fixture since the earliest sets, but a wave of block design later reframed colorless permanents as the connective tissue any deck could run, and a defensive body this cheap to deploy fits that brief exactly. A 0/4 stops most early aggression cold, blocking two-power attackers indefinitely while demanding nothing from the deck around it. The trade-off is total: Defender means it never threatens anything back, so it works only when buying time is the entire plan. As a creature it is also an artifact, which cuts both ways: it counts for affinity, feeds metalcraft, dies to artifact destruction it would dodge as a plain wall, and becomes a target for effects that animate or sacrifice artifacts. That dual identity does the real work. A bare 0/4 blocker is interchangeable, but a one-mana artifact body that happens to block is a chassis: it does one job, asks for almost nothing, and slots into any color precisely because it belongs to none.