The River Gate is one of the seven huge gates that surround the huge walls of King's Landing. It is known as the Mud Gate by the population of the city. It is near the center of the southeast wall and connects Fishmonger's Square and the wharfs on the Blackwater Rush. Opposite the River Gate on the southern shore of the Blackwater is the kingsroad, which leads to Storm's End.
In the reign of Maegor I Targaryen, Maegor had his own wife and queen; Alys Harroway tortured to death and executed. Following her death, Alys body was cut into seven parts and mounted on spikes above each of the seven gates of the city, one of which was above the River Gate. 
In the Fall of King's Landing which occurred during the civil war known as the Dance of the Dragons, Thirteen House Hightower knights and a hundred men-at-arms managed to hold the River Gate for eight hours from both attacks without and within the city. Despite this impressive resistance, the other six gates fell swiftly to the Blacks. 
Later the same year, a massive riot broke out in the city, Sailors unable to return to their ships attacked the River Gate. By morning the bodies of the captain of the River Gate and three of his serjeants hanged from the gate. 
A Clash of Kings
The commander of the River Gate, Ser Jacelyn Bywater, is made the new commander of the City Watch of King's Landing. The River Gate is assaulted with a battering ram by the army of Stannis Baratheon during the Battle of the Blackwater.