I'm looking for a fantasy book that is roughly the contemporary of Piers Anthonys Incarnations Of Immortality series. There is a castle or manor that was enchanted maybe by the last owner/wizard. People can come in but no one leave until someone solves the riddle and finds the literal key to the building. At which point, the finder inherits the castle and a lot of power and everyone can leave. People don't age or age slowly. I think that some lady turns into a dragon. A poor man with some magic is stuck in the castle because he thought with the wealth he could court the noble girl he loved. It's a girl who solves the riddle of where the key is. She pulls out a dagger or knife and cuts into her heart from which she takes the key.
Thank you for your help.