Highgarden Castle is the seat of House Tyrell, regional capital of the Reach and heart of chivalry in the Seven Kingdoms. It lies on the Mander and there the Ocean Road meets the Roseroad, making it an important crossroads.
Margaery Tyrell describes the castle as having groves and fountains, shady courtyards, and marble colonnades. It is filled with singers, pipers, fiddlers and harpers. The stables have a fine selection of horseflesh, and there are pleasure boats to sail along the Mander.
Highgarden was once the seat of House Gardener, until Aegon the Conqueror defeated King Mern IX Gardener and his host on the Field of Fire, ending their line. When Aegon then marched his host to Highgarden, its steward, Harlen Tyrell, surrendered the castle. For this, Aegon has given him the castle and gave the House Tyrells dominion over the Reach.