Sanctuary Wall

A plain tapper buys one turn: the creature stands up on its controller's next untap. Bolting a stun counter onto that tap turns one turn into two, because the untap step gets eaten to peel the counter instead of straightening the creature. So the ability is a two-turn denial on the thing that matters most, an attacker held off the crackback or a blocker pried out of the way across a full turn cycle. The cost is the symmetric clause: this creature stuns itself whenever it stuns something else, which is not free at all. A stunned Wall cannot block on the crack-back (the exact job a 0/4 Defender exists to do), and the self-stun locks the activated ability offline for that whole cycle, so each use trades away a turn of the body's defense and a turn of the engine's uptime. That friction is baked into the -plus-tap price: this is a grind piece meant to pick apart one threat at a time, not a proactive tempo swing, and the four toughness is there so it survives long enough to keep grinding. White has printed defensive walls that swap combat presence for a repeatable disruptive effect for as long as the color has had walls, but the stun counter gives this one a bite the old chump-and-gain-life rattlers never had: it does not merely delay a creature, it removes two of that creature's turns from the game.