From A Wiki of Ice and Fire
khalasar marching east from Pentos across the through the Flatlands
The Flatlands is a region on the continent of Essos. The Flatlands are bordered by the upper reaches of the River Rhoyne to the east, the Velvet Hills to the north and the Golden Fields to the south-east. Tillers and toilers who are bound to the land dwell in the Flatlands. The Flatlands is a fertile area of open fields and plains, located around the Free City of Pentos and responsible for feeding it. Many magisters and nobles of the cities have estates on the Flatlands. Illyrio Mopatis owns some orchards, farms and mines in the Flatlands, though he seldom visits them.
The Dothraki have easy access to the Flatlands and find riding across it easy, so Pentosi nobles pay them heavy tribute so as to ensure they do not loot or sack any of the estates or villages in the area.
The Flatlands used to be part of the southern kingdom of the Andals before they fled to Westeros several millennia ago.
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