Brightwater Keep is the ancestral seat of House Florent. The lord of the castle at the start of the novels is Alester Florent.
Following the Battle of the Blackwater, during which House Florent fought for Stannis Baratheon, Brightwater Keep and all of its lands and incomes are granted to Lord Mace Tyrell's second son, Ser Garlan Tyrell, thus establishing House Tyrell of Brightwater Keep.
Brightwater Keep is located near the source of the Honeywine river. It is north of Honeyholt and Oldtown, east of Bandallon and west of Highgarden.