Mine love having a second level, but you definitely need to think of safety when looking at putting in stairs.
This is a photo of mine (taken when I only had my precious Amber). As you can see, i put a banister on the side of the staircase and also a box surround at the top of the staircase so the piggies can't fall.
I also had a plank fitted in at the top so that I can remove the box surround, pop the plank in and seperate it into 2 cages. The stairs at the bottom can also be unlatched and slid out.
Makes it easier to clean out this way, but also handy if i ever need to seperate my piggies for any reason.
I also have a cat, but this wire mesh is very strong and has small squares so she would have no luck getting in even if she wanted to (which she doesn't try to do, my cat is excellent around my guinea pigs). I think this is a similar mesh used for outdoor bird avaries, though my piggies live in my front room, you just never know with cat instincts so i don't take chances even though my cat Riley is very good with them.
My OH built this for me by hand, and it also has storage cupboards underneath it. It was all vanished as well (with pet-safe waterproof varnish).