The city was founded at the end of the Bronze Age by the Hittites, who built a small fortress against the Kashkäer and called it Hakmiş (Khakm(p)is). The well-preserved castle Amasya Kalesi, also known as Harşena Castle, lies about 250 m above the city, on the mountain Harşena.

The fortress stands on the foundations of the small Hittite fortress and in parts still has Hellenistic masonry. In its present form, however, it is largely Byzantine. The town and castle were surrounded by a wall, some of which is still well preserved today.

Amasya Kalesi was destroyed in most historical battles and was subsequently rebuilt in altered form. After the Turks conquered Amasya in 1075, the castle was completely renovated. It was used until the 18th century, when it lost its military importance.