You find this bunker at the Breullaan in Driebergen, nearby the road between Driebergen and Zeist, at the railroad crossing.
At the end of 1943 beginning 1944, the bunker was built to use as a Gefechtsstand for the XII. Fliegerkorps of the Luftwaffe. Because of the many re-organizations and structure changes in the Luftwaffe at that time, the purpose of the bunker alsochanged .So the bunker has never been really used by the Germans.
After the war the Dutch Air-force took over the bunker and situated their Tactical Air Defense Command here till November 1991. In 1995 a multi-media company took over the building and after a renovation of the in- and outside, they have their offices now in a "solid" building.
The following sources I used and I would like to invite you to read them too, because they tell much more about the bunker Cäsar and its function, techniques and the organisation of the German airdefence system.
Specially about Cäsar (English) http://www.gyges.dk/Type%20Caesar.htm
German Air Defence system (English) www.gyges.dk
Specially about Cäsar (Dutch) http://home.hetnet.nl/~witie/Driebergen.htm
1x Luftwaffe bunker, code-name Cäsar.