Wall of Air

A 1/5 in the sky for three mana set the rate that every blue defensive flier inherits, from Wall of Denial onward. The pairing of Defender with Flying solved a specific problem the earliest blue decks had: ground walls stopped Hill Giants, but the air was open, and blue's evasive threats had no symmetric answer when an opponent's flier showed up first. The double-blue commitment is the discipline behind the body: it locks the card to mono-blue or heavy-blue manabases and stops it from being a universal splash, which is what lets the toughness sit at five without warping early defense. The body itself is a clean statement of blue's color-pie position on creatures in 1993: small offense, large defense, and the air as blue's domain by default. Later sets refined the shape (granting reach to green walls, granting tap abilities to utility walls, eventually breaking Defender entirely with cards that attack as though they did not have it), but this is the reference point, one of Alpha's original Walls and the one whose design template the game kept returning to. What it does is unremarkable by modern standards; what it established is that blue gets to wall off the sky as a baseline, and that assumption has shaped three decades of creature design.