ARX, in all ancient cities, was a fortified eminence or rock, either within or close by the city itself. The arx at Rome was a part of the Capitoline Hill, the other part containing the Capitoline temple. In Greek towns the arx was called ἄρκα or ἀκρόπολις, and in several of them it bore a distinct name; whence at Thebes it was called the Cadmea, at Argos Larissa, at Corinth Acrocorinthus, and at Athens, sometimes Cecropia, or simply ἡ πόλις. The modern name citadel does not correspond with the term arx in every respect, the latter being always situated on an eminence. (L.S.)
ARX also denotes a consecrated place on the Palatine Mount, where the augurs publicly performed their office. Some suppose 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; and is the same as 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 herbs used in the ceremony of making leagues and treaties.