Akershus Fortress started as a Mediaeval castle in 1299 by Håkon V Magnusson, and 300 years later it was developed to a fortress with cannon. It includes Akershus Castle and church, several museums and part of it is still used by the military. It is placed centrally in Oslo in a beautiful green nature area, close to Oslo Town Hall and with an excellent view over Oslo and the fjord.
More information can be found: here, here and here in English + several other sites.
The micro cache is long black cylinder containing a combined micro stashnote/log and pencil (to stay). Please don't reveal the hiding place in your log comments
There is a new path going to the cache area now. Please use this to get close to the cache site!