I agree with MissDonna. It is perfectly fine for one to respond with their opinion but there are different ways to say it. It is like when people talk to you in real life, they could say something in a way that makes you feel offended, but they may be able to say the same thing in the way that you understand without offense. And some of these comments did sound rather. . . attacking. [Although the response wasn't really any better
]
As for ventilation, maybe the glass could be replaced with meshing like the kind used to cover bin cage windows? That might help the ventilation big time.
Also, if the hammy is potty trained then pee everywhere shouldn't be a problem (right?). However I personally would cover the bare wood with something, even if its not the substrate. I cover my wood platform in ripped toilet paper and hay, so if she does get a weak bladder up there, at least it is absorbed enough where it is not a problem. Also, I've seen a lot of people plastikote (or what is it called?) homemade wood products, so that could be another possibility.
I don't know how to take care of elderly hamsters so I certainly don't know for sure about the climbing issue, but as long as we are comparing hamsters to people, I have two grandparents near the same age - one cannot get around very well, and therefore cannot comfortably use stairs and such, but the other is just as fit and able as my parents are (sometimes more so haha). So if every hamster is different like people are, then maybe Charlie's hamster can indeed still enjoy climbing. As long as they keep an eye on the hamster to make sure it's not struggling. If it does, then, yes, it should be moved to a large single level cage.
Although, with all those levels, I would also include a second bottle on the bottom level, just for ease of access to water which is a necessity to all living things.
All in all, the consensus everywhere (not just on this forum) seems to be 'the bigger the better'. Also, you should not add together levels to the total floor space. So that should definitely be kept in mind!
Overall I think it is super cute! Although in my personal opinion, a little impractical. I would love to watch my hamster explore and play in this cage, but I don't think I would house her there.