my girl spent more time at the bottom a few days before, went into labour at the bottom, then just before squeezing the baby out, she ran all the way up and delivered in the snug. however, as soon as she checked the baby was ok, she went back to the bottom of the cage. after a minute i grabbed the baby and put it with her - she then cleaned it off. i thought she was having more, sometimes they clean them all together so we had a trip to the vet two hours later when no baby appeared (she wasn't having any more).
i do think that if i was not there, the baby would possibly not have made it because she didn't clean/dry her.
i would suggest you take shelves out so you only have a single level cage. i tried moving fluffy to a single level cage but she got so stressed out that i decided to leave her alone. i asked for advice and somebody said they have babies delivered in a colony cage all the time without any problems. she'd also had triplets in that cage before (we didn't know she was pregnant then).
but, a single level cage prevents mum from accidentally jumping on top of the kits, she can't run away from the kits when they want food and she's there to keep them warm. i would do this if you can.