We set off from Poyrazlar Lake and pedal to Akgöl, a freshwater alluvial embankment lake, to travel between Karasu and Ferizli districts in Sakarya. The lake was formed by the alluvial deposits accumulated by the Sakarya River, which passes about 2.5 kilometers away. The villages of Gölkent, Kuzca, Bakırlı of Ferizli and Adatepe, Konacık and Kancalar of Karasu are located on the lakeside. You can top up on your basic needs in the grocery stores and village coffee houses here. Although the size of the lake is 19 km², most parts of it are reeds and swamps. As you reach Akgöl, there is a historical point which you might want to visit. Harmantepe is a Caucasian (Abaza) village 13 kilometres from the centre of Adapazarı. There seems to be no altitude difference on this route. There are tours available for those who are curious about Küçük Akgöl and Harmantepe Castle. Harmantepe Castle, located to the north of Harmantepe Village in Adapazarı, is built on a small natural hill. The surrounding area was originally wet and swampy, but thanks to the digging of canals on both sides, it has been transformed into fertile agricultural area. In order to reach the castle, it is necessary to deviate from the road leading from Küçük Söğütlü to Akçakamış Village to the field road on the left side. Transportation is a bit difficult in rainy weather, but nothing is impossible with a mountain bike. Harmantepe Castle, built in the 12th or 13th century to protect the eastern border of Byzantium, is one of the defensive structures, which are outposts and watchtowers facing each other along the western bank of the Sakarya River.