Harmantepe Castle is a Byzantine fortress near the village of Harmantepe in the Söğütlü district of Sakarya, built in the twelfth or thirteenth century to defend the eastern frontier of the Byzantine empire. It stands on a small hill beside the stream running out of Lake Taşkısığı.
The walls are ten metres high and two metres thick, with six towers of five by five metres along them. Stone from the surrounding area was used; there is no brick, only rubble stone and lime mortar. The castle has five gates, the main one on the south side. Long unrepaired, its walls, gates and towers have been badly damaged, and years of neglect have left it covered in vegetation.
Standing in the broad Adapazarı plain, the castle is exposed, and the traces of a ditch thought to have been dug around it to strengthen its defence are unclear. Its construction matches that of Seyifler castle. Its date has been disputed: some researchers connected it to the ancient canal project of the sixth century that also gave rise to the Bridge of Justinian, but the finds from the castle have established that it belongs instead to the line of fortresses built west of the Sakarya river against the intensifying Turkish raids.
It is the soundest Byzantine castle in the province of Sakarya, and a decision has been taken to restore it and open it to visitors.
The castle was registered as a cultural property to be protected by decision of the Bursa conservation board in 1992, and the directorate counts it among the best preserved castles of the lower Sakarya basin. One detail repays a close look: inside the lower part of the bastions, timber beams were laid in a grid through the rubble-and-lime fill to tie the masonry together; the timbers rotted away over the centuries and the round holes they left can still be seen, with arrow slits above them facing several directions. Of the five round-arched entrances the main gate is on the south, and the sockets survive into which the wooden bars that barred them from inside were pushed back into the wall.
As of 2023 the directorate recorded cleaning and excavation under way and the castle closed to visitors, so it is worth checking before setting out. The last two kilometres of the approach are unsurfaced but passable in any season.
Getting there
The castle is reached from the centre of Söğütlü by 2.5 kilometres of asphalt road followed by 2.5 kilometres of unmade road.
When to go
- Climate
- Across Sakarya as a whole, monthly mean temperatures run from 6.3 °C in January to 23.6 °C in July; the mean stays between 15 and 25 °C in May, June, July, August, September and October; rainfall is heaviest in December (106 mm) and lightest in August (48 mm). Long-term means for 1951-2025 from the Turkish State Meteorological Service (MGM).
