Mantarys is the only overland waystop of any note for travellers and merchants taking the land route from the Free Cities to Slaver's Bay.
In the books
In A Song of Ice and Fire, Mantarys used to be a city of the Valyrian Freehold. Although it survived the Doom of Valyria, it has since gained a foul reputation as a city of "monsters". The road from Volantis to Slaver's Bay is called the "demon road" and is shunned by most travelers, who prefer to take the sea route south around the Valyrian Peninsula. It is not considered to be one of the Free Cities, likely due to its isolated nature and unpleasant reputation.
Mantarys is the principal surviving city of the Lands of the Long Summer, an area lying immediately north of the Valyrian Peninsula which used to be one of the heartlands of the empire.