A guardhouse like a Roman fort
Schinkel's first major building in Berlin was a guardhouse for the Crown Prince's Palace, with a square plan inspired by a Roman military camp and a Doric portico in front.
A small temple-like building on Unter den Linden has been a royal guardhouse and four different memorials. Inside, there is a single sculpture. Here's what you're looking at, and what to listen for when you get there.

Schinkel's first major building in Berlin was a guardhouse for the Crown Prince's Palace, with a square plan inspired by a Roman military camp and a Doric portico in front.
It became a memorial to the dead of World War I in 1931, East Germany's memorial to the victims of fascism and militarism, and in 1993 Germany's central memorial to the victims of war and tyranny.
Inside, Käthe Kollwitz's "Mother with her Dead Son" sits beneath an opening in the roof, so rain and snow fall on the sculpture.
Germany's central memorial to the victims of war and tyranny, in a former royal guardhouse by Schinkel.
It's an enlarged version of Käthe Kollwitz's "Mother with her Dead Son".
Yes, it's free to enter.