From someone who formerally lived in Kassel, I can recommend these. Both are 4-star near the castle, the museum, the bathhouse and the Wilhelmshoehe train station. You must visit the Kurbad bath-house no matter what.
Schlosshotel Wilhelmshöhe
Schlosspark 8
34131 Kassel, Bad Wilhelmshöhe
Telephone: 0049-(0)561- 30 88 0
Fax: 0049-(0)561- 30 88 428
[email protected]
http://www.schlosshotel-kassel.de
Kurparkhotel
Wilhelmshöher Allee 336
34131 Kassel, Bad Wilhelmshöhe
Telephone: 0049-(0)561-318 90
Fax: 0049-(0)561-318 91 24
http://www.kurparkhotel-kassel.de