The Outcast, by Louise Cooper

Second paragraph of third chapter:

The shock had caught him off guard, but now he was rallying his composure – although it took considerable effort in the face of what had happened. No human being should be capable of breaking through the barrier which held this Castle frozen in a Timeless limbo. His own power, great as it was, couldn’t penetrate the formless, dimensionless yet appallingly real warp of time and space that had trapped him here in his last, desperate attempt to save his life and his soul; and whatever he psychic talents, Cyllan was no true sorceress. Yet she was here, as real as he was…

Second in Cooper’s Time Master trilogy, almost entirely set in and around the castle where her protagonist is being held captive and from which he is trying to escape. The really subversive bit is that the protagonist is very clearly the Bad Guy, and his freedom could lead to disaster for the rest of the fantasy world; Cooper shows this pretty clearly, but also engages our sympathy very successfully on behalf of the villain. A very strong story. You can get it here.

This was my top unread sf book. Next on that pile is The Cartographers, by Peng Shepherd.