It would be unfair to specifically target Twycross with this criticism, but if we ever need a picture to illustrate why keeping primates in open-topped enclosures can be an inefficient and/or poor use of space then we have a winner!
The mind boggles, doesn't it? Kudos for the new development, of course, and the inside looks great, but using lightweight mesh would have resulted in enclosures that were not only spacious, but also sightly, safe, and species-appropriate.
@Giant Panda Yes, think how much more space they would have if this was a giant aviary-style construction. I quite like it as it is but that would have been a big improvement. One little thing- some small platforms at the tops of the poles might allow the gibbons to sit up on top without having to balance precariously on them.