Machaerus (meaning “black fortress”) is one of many strongholds established by Herod the Great, near the Jordan Valley and the Dead Sea. Periodically the site has been excavated and explored, from the late 1960s to the present day, revealing significant moments in its history (from 2009, excavations have been undertaken by the Hungarian Academy of Arts, in collaboration with the Jordanian Department of Antiquities). We've found that Machaerus was heavily fortified, but also served as a royal citadel; it was protected by high walls, and built atop a hill with steep cliffs alongside.
The site was originally built by the Hasmonean king, Alexander Jannaeus, to defend his eastern territory of Peraea against the expansionist Nabataeans; and although the Hasmoneans fortified the site around 90 BCE, Gabinius later destroyed it in 57 BCE. However, the fortress was eventually rebuilt by Herod the Great in 30 BCE to serve as a military stronghold and summer residence. Herod’s fortress measured 110 m east–west and 60 m north–south. It was divided into two parts: The eastern part contained elements of a bathhouse, paved with mosaics; the western part had a peristyle court. The site commanded an expansive view of the Dead Sea region, crucial to its function as a strategic outpost.
The fortress was later inherited by Herod the Great’s son, Antipas, in 39 CE (during which time, the story of John the Baptist’s imprisonment takes place), and then was again passed to Herod Agrippa upon Antipas’ banishment in 39 CE. Soon after this, the fortress fell to Roman control, and was then seized by Jewish rebels. In the ensuing exchange between the Romans and the Jewish rebels, the fortress was ultimately torn down to its foundations, leaving the extensive ruins we see today.
The Archaeology of Roman Palestine. Chancey, Mark A; Porter, Adam. Near Eastern Archaeology 64.4 (Dec 2001): 164-203.