in the ancient military art, a town, fort, or castle, for defence of a place.

The arx in ancient Rome was a distinct edifice from the capitol, though some have confounded the two. According to Ryckius, the arx, properly speaking, was a place on the highest part of the Capitoline mount, stronger and better fortified than the rest, with towers and pinnated walls: in which was also the temple of Jupiter Capitolinus.