The Kastellet can be visited
Kastellet Copenhagen

In the northeast of the Danish capital, Copenhagen, lies the Kastell of Copenhagen, a historic fortress. The complex, completed in the 17th century, is distinguished by its pentagonal shape and a system of moats.
History+
The origins of the Kastell date back to 1625, when King Christian IV had an initial redoubt built at the harbor entrance. Following the loss of territories east of the Øresund in 1658, this fortification gained strategic importance. Subsequently, the Dutchman Henrik Ruse, who had previously contributed to the expansion of Harburg Fortress, was commissioned to expand it.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://www.wehrbauten.de/thread.php?threadid=1461
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 55.6914, 12.5943