The Aachener Weiher isn't anything special, per se. It's just a little concrete pond with a bit of park around it. You can't swim in the pond, and it's close to the Aachener Straße, which can make it a bit noisy at times.
But what makes the Weiher special is its location - it's situated between the Rings and the Innere Kanalstraße, an easily accessible location quite in the heart of Cologne. It is also located close to the university, with only a small street separating in from the Uniwiese behind the University's main building.
Thus, as you might imagine, it's one of the prime spots in Cologne for people wanting to have a barbecue on a lazy summer evening. This has advantages and disadvantages - both being there are lots of people.
It's nice to hang around among people, with the occasional barbecue grill lighting the area in soft, warm tones as the day tends to be more of a night. Lots of young people around, if you want to meet some. Also, there's the typical wandering parties like a bachelor party, which have a large audience to choose from for their drunken games.
The downside is that there are many people: if you want to have a quiet evening in the park with some friends, relaxing, you might be better off in one of the more remote spots. It tends to be noisy all day round (traffic and people).
If you're rather one of the pub types, there's a biergarten for your need of crampiness, if you like. read more
