Telos was originally an ice planet in which the Cryons could live. The Cryons were later driven underground the surface temperature rose. The Cybermen, after the destruction of Mondas and being driven from their worlds by the Alliance, conquered Telos and the Cryon cities around the 23rd century.
The cities were used by the Cybermen as a cryogenic facility. When the second Doctor, along with a party of archaeologists, visited Telos in the 26th century they found an arid, mountainous planet. Under the surface, however, lay the tomb of the Cybermen.
The sixth Doctor visited Telos and helped one of the Cryons to destroy the Cybermen allowing the Cyrons to retake the planet.
Additional information about Telos