The Gök Medrese Cami was built from 1266 to 1267 for Seyfettin Torumtay, the Seljuk governor of Amasya. The eyvan (vaulted recess) serving as its main portal is unique in Anatolia, while the kümbet (domed tomb) was once covered in gök (sky-blue) tiles, hence the name.
A full restoration of the mosque began in 2017 and was still ongoing during our last visit, though was nearing its final phase so check if it's open again when you're in town.