The Büyük Ağa or Kapı Ağa Medrese is located in the city center of Amasya, Turkey, on the north bank of the Yeşilırmak, just across river from the Bayezid II Mosque complex. It was constructed in 1488/894 AH at the order of Hüseyin Ağa, the senior officer and head of the white eunuchs at the Ottoman court (Kapı Ağa or Büyük Ağa in Ottoman Turkish).

The madrasa is notable for its octagonal groundplan. The exterior has eight facades, four nearly aligned with the cardinal directions, and four angled to the northwest, northeast, southwest, and southeast. One enters the complex via a portal on the northwestern facade. The portal gives onto an iwan that leads onto the central courtyard, around which wraps an octagonal portico.

The portico is composed of domed bays, three on each of its eight sides. Through each domed bay on the porch is a door leading to a square chamber. These chambers are also domed and are lit by windows cut into the outer walls which they form. The exception is the northwestern side of the building in which the entrance iwan is flanked by two domed cells, and the south side, where the portico gives onto a larger prayer hall. The prayer hall is shaped like a T: a central domed cube on whose south wall is a mihrab, with two small extensions on the north end covered by half domes.

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