Still working on the castle conundrum? Maybe this will help.
It's guarding a concentration camp.
More: Alert reader Talya Leodari correctly identifies the castle in the comments. The Alte Feste was an German fortress built in 1890 in Windhoek in what was then German Southwest Africa.
The flag is the Black, White and Red tricolor with the imperial German eagle in the center.
The equestrian statue shown in the previous post is of a mounted infantryman of the German SWA Schutztruppe or "Protection Troop". It stands in front of the Alte Feste, which today houses the national museum of independant Namibia.
The concentration camp is for captive Hereros following their defeat and decimation in 1904 in the months following the Battle of Waterberg on August 11th, 1904. You can read more about this dark chapter in Africa's colonial history in a post of mine from last year on the topic.