Corvettes, frigates, and sloops by construction standard

Was the Castle class a corvette or a frigate?

It was a corvette, built closer to commercial standards; frigates sat a step higher and sloops used full naval standards.

The Castle class remains a corvette, while River-class ships are frigates. Corvettes sit near commercial construction standards so civilian yards and practices can contribute readily to production. Frigates occupy a commercial-plus middle ground, with more naval features and capability. Sloops sit at the naval-standard end of the continuum. Machinery arrangement and screw count help distinguish the types, showing why size or mission labels alone do not settle the classification.