So this was how I discovered him prior to layout change (he's the Champers), I have zero idea how he managed it as I deliberately made that bit "too hard for him":

My version of a "HLD friendly" (yes, I am mean.) layout:




I really wanted to make the cage more accessible and safe for Chino as he's a pretty determined little chap, but I'm also very much aware of the fact I have 5 other rats that appreciate a challenge so I've tried to accommodate them all in this layout.
I've found the yellow tube to be fantastic for him as he can drag his butt up it reasonably easy and sorta... slide back down when he wants. Although he's mainly just using the bottom of the cage and the hammock + litter tray now, he has been right to the top and slept on the blue radiator bed a couple of times and theres enough stuff to catch him if he falls now. If/When he gets to the point he cant climb up the tube anymore then I'll add a radiator bed or similar in the bottom, but for now I'd rather encourage him to climb up there while he still can.