Its true Drills and Mandrills are esentially terrestrial in the wild, but in zoos they tend to use all levels in the more limited space at their disposal. The Drills at Port Lympne have one of the older-style tall cages and they scuttle about along the topmost branches and beams as much as they use the ground. So I think the height/level of access between indoor/outdoor in the Edinburgh house is largely irrelevant.
More important perhaps is whether the house as a whole is still adequate for the occupants in this day and age, as its one of the few 'older style' Monkey Houses left in a mainstream UK Zoo. Hopefully revenue from the Pandas etc will allow for a revue/improvement/new build(?) for this house at some stage soon, as Edinburgh houses an important and eclectic Monkey collection- particularly the Drills and the several African Guenon species, which still breed successfully, despite the rather outdated surroundings.
Just thinking of the age of the Edinburgh monkey house, I think it was built in the early seventies, making it about forty years old, although it is starting to look a little dated I would say it is still up to the job, there are other exhibits in need of updating at this zoo before the monkey house, all in good time!