In my setting the entire world is a vertical wall, so I've been thinking about what might pass for a watchtower in such a setting, and come to the idea of a watchcantilever, a "horizontal tower" of sorts, a protruding (most likely triangular or half-arch styled building) structure with a small outlook post on the end, where one or two people are stationed to watch the surrounding cliff sides for travelers or enemies.
But I'm unsure that one with a sufficient length can be built to be used as an observation post. There's The Leaning Tower of Lire problem that gives some insight, but it focuses on an independently laid unsecured stack of blocks.
So, using anything that predates the invention of steel construction (steel cables and girders can allow you to reach pretty far I'd imagine), what's the longest structure protruding from a cliff wall can be achieved? Assume no fantasy elements are involved.