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.

Arx also denoted a consecrated place on the Palatine Mount, where the augurs publicly performed their office. Some will have the arx to have been the augural temple; but Varro expressly distinguishes between the two.

Arx was particularly used for a public place in Rome, set apart for the operations of the augurs. In which sense arx amounts to the same with what is otherwise called auguraculum and auguratorium, and in the camp augurale. Out of this arx it was that the feciales, or heralds, gathered the graft used in the ceremony of making leagues and treaties.