Duskendale is a port town in the Crownlands which answers directly to the King on the Iron Throne. It is located north and east of King's Landing on the west coast of Blackwater Bay, east of Rosby. It is the seat of House Rykker.
In the books
In the A Song of Ice and Fire novels, Duskendale is a large town of several thousand inhabitants. During terms of crisis, it serves as an auxiliary port for King's Landing, being the only other port on the east coast of Westeros able to handle ocean-going ships south of Gulltown. The town is built around its harbor and has cobbled streets. Chalk cliffs lie to the north and low limestone hills shelter it to the west. Fishing villages dot the coastal road for miles in either direction. South-east of the town, a large rocky headland shelters the town from the worst of the storms coming in off the Narrow Sea. The Lord of Duskendale resides in the Dun Fort, a large square keep with drum towers, which overlooks the town.
In the early 270s AL, Lord Darklyn of Duskendale began to resent the amount of taxes he had to pay to King Aerys II Targaryen. He withheld his tax revenues. Aerys II, resenting the growing perception that he could not handle a crisis without the help of his Hand, Lord Tywin Lannister, decided to settle the matter personally. Due to incompetence, he ended up a prisoner of the Darklyns and spent several months in their dungeon until he was rescued by Ser Barristan Selmy of the Kingsguard, whilst Tywin commanded an army in besieging the town. Having been rescued, Tywin ordered that the Darklyns be utterly annihilated as a house, with several of its leaders burned alive on a pyre outside the town. The Hollards, the senior-most house of the town after the Darklyns, suffered the same fate, although Aerys spared the life of their youngest boy, Dontos Hollard, at Ser Barristan's request. House Rykker was given the lordship of Duskendale following this crisis.
This incident, known as the Defiance of Duskendale, is said to have marked the beginning of Aerys's descent into madness. The term 'Mad King' came into common usage after his incarceration and Aerys's distrust of the Lannisters became more pronounced after this time.
- Duskendale at A Wiki of Ice and Fire (spoilers from the books)