The Cyclopean Mountains are the mountains set between the Mohaine Desert and the Western Sea. These are the mountains that Roland Deschain and Jake Chambers travelled through in their attempt to catch the Man in Black.
Jake and Roland made their way up into the mountains and progressed up steep ridges and into valleys between the peaks. Eventually they came to see the Man in Black standing upon a cliff above the travelers right by the mountain's entrance. He taunts the two travelers and Roland shoots at Walter but missed (this was likely some dark magic as Roland is always very accurate). The Sorceror then fled into the mountain, to be followed by the two companions.
Below the Cyclopean Montains there is little to no light, mostly darkness; wet, cold, damp darkness. However, there is an old rail that Jake and Roland follow. Eventually they came across an old mine cart and rode it through the darkness.
There is also a very old terminal of sorts found in the darkness built by the Old Ones. At the terminal there were still lights and switches that still worked, Jake and Roland found a weapons kiosk with old guns and a rotten bow. They also found another old kiosk with a corpse that has been mumified. In its hands was a newspaper that crumbled to dust when touched. Roland commented that his death is likely due to the gas weapons of the old people.
There is no sign of life below the Cyclopean Mountains save for the slow mutants; terribly ugly humanoid creatures that attack Roland and Jake as they made their way through the darkness. At one point they almost captured Jake and ripped him from Roland's grip. They are just smart enough to place rocks on the tracks of the old rail to stop the mine cart from moving.
There is also a river flowing underneath the mountain.