One of the most important sites is the ruins of a medieval castle, is one of the most notable treasures of history and is very well preserved. It is located on hills next to Haffner Street , 400 m from the beach, which were formed during the last Ice Age. It is surrounded by steep cliffs and deep valleys with streams. It was defended from the west by a moat and dike in the shape of a horse shoe. No moat was detected from the sea site; most likely a swampy bank existed during this time making it impossible to attack from sea.

The first archaeological survey was done by German archaeologist H. Conwentz in 1885. Because of his efforts, the site was bought from the city and declared as a protected area. The next excavations were done by W. La Baume in 1934 and were sponsored by the Museum of Natural Science and Archaeology in Gdansk . All antiques and documentation were lost during WW II. After the war, it wasn't until 1961 when first excavation was done again. From 1961 - 1966 a major excavation was sponsored by the Archaeological Museum in Gdansk and carried out by A. Lukowa and A. Szymanska. Later in 1995-1966 and in 1999 more work has been done to uncover the town's history under the guidance of A. Szymanska. During the excavation, the most significant part of the dike and main living area was explored. It has been discovered that two phases of settlement existed. From the 8th to the mid 19 th century, a small open settlement was established, and later it was transferred to a medieval castle surrounded by a defensive wooden and mud dike with heavy stones for extra support. On the top of the dike the remains of a burnt wooden construct was found, which served as a defensive reinforcement. They were most likely the remains of a defensive tower, whose role it was to control and observe the sea shore. Its existence lasted until the construction of the medieval castle in Gdansk in the 10th century before the Piast era, when the family of the first Piast king ruled Poland . During it short existence the settlement was razed three times and rebuilt until it was finally abandoned or destroyed in around the 10th century. Because of the discovery of the main gate, which was supported by stone wall and homes, some mainly wood homes were partially buried in the ground next to dike, and the layout of the settlement was accurately reconstructed.

Countless artefacts that have been discovered such as glass, ceramics, clay items, amber necklaces, and iron and bone items give us a good idea how those early inhabitants lived. Glass bead necklaces, which were imported from places like Byzantium, Nordrhein (Germany), Haithabu (Dutch), Birka (Sweden) or Old Ladoga (North Russia) indicated that trade was well established with almost every part of the Europe. Animal (pigs, sheep, goats, horses) and fish (cod, herring, sturgeon, carp, pike, and pike-perches) bones which were found indicate that farming and fishing also played a major role in the economy of those early settlers. Bones of the grey seal were found too, which lived in the waters of the Gdansk Gulf during this period. Old Sopot medieval castle became an Archaeological Museum with wide recognition across Europe due to sponsorship of the Gdansk Museum and authorities of Sopot. The entire settlement was reconstructed, the gate, houses, and palisade were rebuilt in the exact place where they once stood. All details are consistent with our knowledge of similar objects from many early drawings which came from other parts of Europe like Russia or Germany , which have been preserved to this day. Because of its location in the center of a very busy tourist resort, the medieval castle is not only a very interesting place to spend some time but it also promotes the history and artefacts the early culture of East Pomeranian. During open season you can see here presentations of the craftsmen, medieval tournaments, and the presentation of various weapons like bows, swords, spears, and battle-axes. You can also have on hands practice, and instruction on how to use these weapons. Also, many schools take their students to display a presentation of live history here. There are many more attractions like parties and meetings held the atmosphere of this rich history. This area attracts many groups of people, particularly students who are heavily involved and voluntary work to make this place more attractive.