They should love climbing on the bars then. I would advise taking out the ramps to the levels.. They can easily climb the walls to get where they want to go and it gives them more exercise than using the ramps (the ramps also take up lots of floor space).
If you are used to using litter, be warned, the pans in the DCN are very shallow, too shallow for litter. I had to replace mine with the stainless steel pans from Bass Equipment. They were expensive, but worth it IMO. If you try using fleece liners, use something absorbant below them to catch the urine and be prepared to change them often... I had to change mine every 3-4 days and that was with 3 rats in a DCN. I switched back to aspen in the Bass pans and have no smell issues now.