Santa Maria del Rosario nel Castello was a garrison church or chapel, now demolished, in the outer circuit of fortifications at Castel Sant'Angelo.
The pentagonal outer ring of defence was completed in the reign of Pope Pius IV (1559-65) the architect being Francesco Laparelli. It had to be added in response to the development of cannon technology, which had rendered the previous castle completely obsolete as it stood.
Included in the scheme were three barrack blocks, running parallel to the north-west, north-east and south-east sides of the pentagon. These had pillared covered walkways along their inside frontages. The north-east range had the little church occupying the full width of the block slightly north-west of its centre, and with its entrance off the walkway.
This was the place of worship of the castle's resident army garrison, and was run by a confraternity which arranged suffrages for dead comrades. The first documentary reference in 1768 referst to this. The old castle itself has several other chapels as part of its fabric, but this one was listed by Nolly in 1748 as a church.
The Papal government lost control of the castle in 1849, because the French troops stationed in Rome to guarantee Papal rule were based there. In 1870, after the Italian annexation, the castle was turned into a prison but was decommissioned in 1901. The church had been abandoned before then, and possibly long before.
The pentagonal fortifications had already been partly destroyed for the Tiber embankment before then, but the block containing the church survived until the whole area between the glacis and castle proper was cleared in the 1930's and turned into a public park, the Parco Adriano.