At 2041 mountain time, a Swearingen SA226TC was damaged in a gear-up landing on runway 35L at Denver International Airport. The two pilots were not injured. The crew lowered the landing gear selector in preparation for landing and heard the familiar sound of the gear extended, but the red transition lights remained illuminated, which they said had been a recurring problem with this airplane. They cycled the landing gear but it did not solve the problem as it had in the past. They said they were confident the landing gear was extended because the hydraulic pressure gauge indicated 2,000 pounds.