Morgan tricked Lancelot into her castle. Putting him to sleep with drugged wine, she blew a powder into his nostrils through a silver tube, thus taking away his senses for a time. It seems to have had a curious effect. He did not lose his memory, apparently; seeing a man paint the history of Aeneas, Lancelot was inspired to paint his own life around the walls of his room. He does seem to have been quite content to remain, in effect, Morgan’s prisoner for two winters and a summer.
At the end of this time, a spring rose, in a garden Morgan had planted outside his window for his enjoyment, suddenly reminded him of Guenevere. So he broke the iron bars of his window, plucked the rose, armed himself, and kept on going, the spell broken. He spared Morgan on this occasion for the sake of her brother Arthur.