The moat isn't that deep as i recall but then they would come up against a very high wall before the tropical plants you can see in the photo. Perhaps a regular visitor to Krefeld would be able to enlighten us?
When i was there last year there were a couple of large posters around the zoo advertising various fund raising programmes for just this purpose and the designated area (attached to the ape house) indicated work had commenced. Don't know how far they've got.
Had a look at their website; nothing on there about the new gorilla garden opening yet, although the guidebook for 2012, for which there is a PDF on the website, claims that this will be opening in Spring 2012, with outdoor facilities for chimps and orangs to follow.