Short answers, pulled from the story.
The Subiaco Dams were built as pleasure lakes for Emperor Nero during his reign from 54 to 68 AD. They served an imperial summer residence at Sublaqueum, in the Roman province of Latium, used for hunting trips and banquets. Later, under Trajan, the dammed lakes were converted into water reservoirs supplying drinking water to Rome.
The largest Subiaco Dam had a reconstructed height of 40 metres, possibly up to 50 metres. It was 13.5 metres thick and 80 metres long across the top. This made it the highest dam in the Roman Empire and the tallest on Earth until the Kurit Dam was built around 1350.
The last and largest Subiaco Dam was destroyed in 1305, following a flood. According to one historical account, two monks removed stones from the already dilapidated wall to lower the water level near their fields, causing a breach that grew until the wall collapsed entirely.
The Subiaco Dams were built at Sublaqueum, modern Subiaco, in the Lazio region of central Italy, roughly 75 kilometres east of Rome. They crossed the river Aniene at an elevation of around 410 metres above sea level. The name Sublaqueum comes from the Latin sub lacu, meaning "below the lake."
The Tibi Dam in Spain surpassed the Subiaco Dam as the tallest in Europe when it was completed in 1594, standing 46 metres tall. The Subiaco Dam had held the European record from its construction under Nero until that point.
A painting of the Subiaco Dam survives at Subiaco Abbey, known as the Sacro Speco. It was created in 1428 by an unknown painter, 123 years after the dam's destruction. The image shows simple block masonry with two openings below the dam's crest, and Nero's Villa visible in the background.