Rosby is a settlement in the Crownlands, consisting of a castle and a surrounding town. It is the seat of House Rosby, a vassal house holding fealty to House Baratheon of King's Landing. The castle is located north of King's Landing.
In the books
In the A Song of Ice and Fire novels Rosby is surrounded by a town of daub and wattle huts. It sits on the road from King's Landing to Duskendale and is surrounded by rich farms, which grow a lot of the food that helps feed the capital.
- Rosby at A Wiki of Ice and Fire (spoilers from the books)