In Linnansaari, near the centre of Oulu, you can discover the early stages of the founding of the city of Oulu. The island's landforms and buildings still show traces of Oulu Castle, built in the early 17th century, part of the castle's ramparts and the cellar of the former chancellery building remain today. On top of the cellar is the magnificent observation tower of the Oulu Maritime School, built in 1875, which now houses the Star Tower café, which has functioned as a summer cafe for nearly 120 years. The castle itself was destroyed after a fire broke out as the result of a lightning stire, which caused the gun powder cellar to explode. As a result of the famous "Oulu Big Bang" in 1793, a part of the office building's base, a cellar, a southside wall and a gateway were all that was left of the castle. The observatory for teaching astronomy at Oulu Maritime School was built on top of the office building.
There is a persistent urban legend about a secret passage that supposedly leads from Linnansaari to the town side near the Market Square ans the Radisson Blu hotel, or possibly near Oulu Lyceum. On the other hand, plenty of ghost stories about the castle cellar, once also a prison, and the observatory have originated from dramatic human fates.

More information: Oulu Castle Cellar Museum - The Observatory Café