Castle Gracht was the home of the Gracht family on the planet Tara.
While collecting a segment of the Key to Time, Romana I met Count Grendel of Gracht who, taking her to have injured her ankle, took her back to the castle on his horse. He assured her upon arrival that it was quite "escape proof". In the dungeons, he already had both Prince Reynart and Princess Strella imprisoned.
Romana was put in a cell with the prince and tended to his injuries. There she learnt that she was identical in appearance to the princess. Romana managed to escape from the castle but was recaptured by the Count.
After the plot to stop Reynart's coronation failed, Grendel tried to marry Romana and the prince so that by killiing Reynart he could marry the then-Queen and become King. However, the Fourth Doctor entered and duelled with the Count, forcing him over the battlements and into the moat, where he swam away. (TV: The Androids of Tara)