The Salamandra base (not to be confused with the three hideouts) is a large underground complex on two levels that Geralt finds himself and an ally, either Yaevinn or Siegfried, teleported to by Triss Merigold at the end of Chapter III. It is a series of caves connected by winding corridors. There are some less than structurally sound looking areas that require wooden trusses to support to tons of rock threatening to come crashing down.
It is the home of a kikimore colony, complete with their queen. The Salamandra took it over after figuring out a way to control the insectoids. There is also a circle of elements, the Circle of Waning Power tucked away not too far from Alvaro's Portal on the first level of the complex.