Short answers, pulled from the story.
Roman colonists established the settlement of Cosa in 273 BC on a hill 113 meters above sea level. This location sits 140 kilometers northwest of Rome along the Tyrrhenian Sea coast.
Frank Edward Brown directed the first major excavation campaigns at Cosa under the American Academy in Rome beginning in 1948. These initial digs ran through 1954 before resuming from 1965 to 1972 to trace the city plan and principal buildings.
The Arx citadel sat at the highest southernmost point bounded by town walls cliffs and separate circuit wall. It constituted roughly one-twentieth of total townsite area yet held most impressive remains discovered during early American excavations from 1948 to 1950 including three temples plus the Capitolium reaching fullest development in early second century BC.
Cosa harbor represents earliest known Roman harbor communicating with sea through artificial ship channel protected by massive breakwater. Elaborate channels cut into limestone cliff kept mouth free of sand while five large masonry piers built from mortared rubblework tufa and sherds remain visible in outer harbor today.
Cosa suffered crisis during Roman Republican civil wars in the 60s BC becoming depopulated as large villas replaced smaller farms using slave labor similar to latifundia estates found elsewhere in southern Italy. Final destruction came when Sienese army attacked castle in 1329 claiming banditry justification leaving ruins permanently uninhabited.