Karadeniz Eregli Castle located in Kaletepe Hill,overlooking the city, is 150-160 metres above sea level.The first Acropolis named Herakleia Pontica was established by Klearchos on this hill in IV. Century BC
The Castle was examined and a plan of it issued by W.Hoebfner. However it is known to be built in the Byzantine period in XIII.Century, W.Hoebfner has a non-confirmed claim that it was built by Geneoses.
The castle is composed of rubble stone and brick and has an irregular plan. Besides bricks and mortar in the construction, gray tufa stones were also included. Light red bricks among those stones and dark red bricks are used in the courtyard gates.
The castle consists of the reinforcing towers and the walls surrounding the castle courtyard. The remains of a tower can be observed on the left of the front yard. Remains of another tower on the right side, again draws attention. An ammunition storage was located in the outer courtyard of the castle .There is also an arched gate leading from outside to the inner courtyard . A water cistern with rounded corners showing a plan of 8.00×40.00×6.10 m is placed in a large part of the inner courtyard .The depth of the cistern is 5.00 metres.
Ruins of two rooms on the upper floor of the castle are remarkable. A large section of the roof of this room with brick walls was destroyed. But it is understood from the traces that they were covered over with a vault coming until today.