The Sofa of Reasonable Comfort was a piece of furniture within Castle Tersurus.
The Ninth Doctor had bribed the architect to make it, having travelled back to before he anticipated the Master would arrive to bribe the architect into laying a trap such as the Spikes of Doom. As a result, when the Doctor and his companion Emma arrived at Castle Tersurus to meet with the Master, rather than being subjected to the Spikes of Doom, the couple found themselves sat on the Sofa of Reasonable Comfort. (TV: The Curse of Fatal Death)