Well, it's the tower you can see behind me.
It's the highest and most famous of the towers in Groningen. It's original name is Martinitoren (Martini tower) and it's part of the Martini church. d' Olle Grieze is it's nickname and it means 'the old grey one'. d' Olle Grieze isn't dutch btw, it's Gronings dialect. In dutch they would say 'De Oude Grijze'.