Shadow and light can do extraordinary things. I once saw a perfectly rectangular object spanning a shadowed crater floor in my 4" refractor, it showed up clearly and distinctly at 100X and still looked like an artificial structure at all magnifications up to about 200X, the practical limit for my scope. Seeing was excellent.

The crater was on the terminator, right in the middle of a first quarter moon, and its floor completely in darkness. The "box" spanned the crater completely, from wall to wall, was oriented E to W, and had an aspect ratio of about 5:1, longer in the E-W direction. It cast no shadows of its own, and the visual impression was of a completely artificial structure, like a runway, bridge or a building (covering many square miles!). Absolutely no detail was visible, but its shape, edges and corners were perfectly consistent with a rectangle.

Unfortunately, I'm not primarily a lunar observer, I own no detailed lunar maps, and I can't tell you which crater it was, or any other details. But the moon is a very frequently observed object, and if there were any artificial structures on it that large I'm sure someone else would have seen it long before I did. I never saw it again, and as far as I know, neither has anyone else.

No, it was simply an artifact of natural topography, visual geometry and illumination. Weird shapes are often seen along the terminator, when shadows are long and the constantly changing libration and sun-earth-moon relationship rarely gives us the same conditions twice. Alas, our satellite is a dead world. It is merely a detailed record of a long and troubled past.