Prydain was filled with forests, hills, valleys, rivers, lakes and marshes of many kinds. Its most common tree varieties were oaks, elms, and rowans in the lowlands; and pines, alders and hemlocks in the uplands.
Trivia
Prydain is based on Lloyd Alexander's Prydain, which is loosely based on the geography of Britain, which Prydain is the Welsh word for.
Lloyd Alexander's Prydain is inspired by Welsh mythology, specifically the Mabinogion.
Prydain is ruled by a High King in the books, with Taran taking the position in the series' ending with Eilonwy as his queen.