All of the off exhibit areas of this zoo are usually under the exhibit itself. This makes for a great use of space. Also the noise these apes made was amazing in the off exhibit area!!
The old zoo is indeed located in the city centre, and has a limited amount of space. By placing the indoor exhibits under ground ( for example, the elephant's stable is located underneath the kanguru-exhibit), the found a way to display more animals in a very acceptable manner, on such a small piece of land. But there's also a downside; most indoor exhibits can not be visited, which has been felt severely during this years harsh winter; a lot of animals couldn't be seen in their outside- or inside exhibit, so even the regular visitors didn't visit the zoo much.
But back on the baboon exhibit; yes it's a fine exhibit ( especially regarding the fact it's a seventies-enclosure), but it's very hard to maintain an acceptable group size, so very often it seems overcrowded ( allthough the enclosure still is big enough to give any induvidual animal plenty of space, but it's certainly not an ideal situation at these times. Recently a group of baboons has been moved to osnabruck, so there's more space until the move of the animals to the new zoo, or the next population explosion).