Wall of Water

The five toughness is doing all the work here, set deliberately above the reach of the original burn suite and above the power of nearly every early creature, so the wall genuinely shuts a ground lane. This is how Richard Garfield first imagined blue's combat identity: the color that does not attack, but cannot be attacked through. The activated ability is the quiet half of the design, blue mana (the color of permission) converted directly into combat math, letting the wall trade up against creatures it could not otherwise stop, or chip in as a pseudo-attacker in formats where defenders can be turned sideways by other cards. It is also a window into a design discipline the game has largely abandoned: a vanilla-plus-pump wall, double-pipped into its color, with no card draw, no bounce rider, no scry. The pump cost being a flat single mana (rather than the more common two) is the only sign of generosity. Later blue walls would pick up tap abilities, evasion-granting triggers, or card filtering; this one just sits there and dares the red deck to find a way through. Trace later blue defenders back far enough (Wall of Denial, the various defender-matters payoffs) and you arrive at the same founding proposition: blue's first creature commitment should be a brick wall with a mana sink.