"Planetoid X", as the Sixth Doctor called it in passing, was a dwarf planet in the Kuiper Belt with a rocky surface. Carved into a cliff on the planet was a cyber-citadel in the shape of a gigantic Cyberman's head. It housed a Cyber-Planner and was maintained by cybermats.
Above the planet, 10,000 Cyber-ships were hidden in warpspace, ready to travel back to the end of the Great Cyber War and change the outcome. The Sixth Doctor, having been translocated in time by the Monk to replace the Second Doctor, and Zoe Heriot prevented this by "completing the circle": the Second Doctor, knowing the Cyber-Fleet would never activate, wouldn't become involved, whereas the Sixth Doctor did interfere once he took his place. The Sixth Doctor sent a message to his second incarnation saying "come to planetoid X, last of the Cybermen" and history would be put back on track. (AUDIO: Last of the Cybermen, The Secret History)