Longomontanus is of the variety of large lunar formations called a "walled plain", although it is actually more of a circular depression in the surface.

Because of its location, Longomontanus appears distinctly oval in shape due to foreshortening.

The wall of Longomontanus is heavily worn and incised by past impacts, and the rim is essentially level with the surrounding terrain.

The crater floor of Longomontanus is relatively flat, with a low cluster of central peaks somewhat offset to the west.

[1] By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Longomontanus.