Kastellet Fortress

The Kastellet is one of the best-preserved fortifications in the city of Copenhagen.The building is shaped like a five-pointed star with bastions at its tips. In addition to military installations, it houses a church and a windmill.Although the Kastellet is still used by the Danish military, today it serves primarily as a tourist destination and recreational area. It is located in the northeast of the city,not far from Østerport Station, and is served by bus lines 15, 19, 26, and 1A.Admission is free; the Kastellet is open daily from 6 a.m. until sunset. The grounds include a nature-based park, which is also extensively used by
Copenhagen residents and is home to numerous animals.There are also two smaller museums within the complex. A military changing of the guard ceremony takes place daily at 12 p.m. In the summer, military music concerts are held at 2 p.m., which are also worthwhile.A large concert takes place every year on October 28.Source and further information: mycitytrip.com/kopenhagen/kastellet-2210

The castle is near the Little Mermaid. So when you are there, you can pay a visit to the fort. The fort itself is located on a small island and is arranged in a star shape. You can walk inside as well as "on" the former walls. From there you also have a nice view of the harbor.

Very nice castle, a tour of the outer wall is worthwhile.