So glad you posted this... my two ate exactly the same! They just love it. Jumped on top straight away as you say to demolish the roof first... they were only 7 weeks when they had their first one and being new I was omg how did you get up there... I thought guinea pigs couldn't climb! I actually ended up after they made a hole in the roof turning it upside down so they then started on the inside and walls!
I'm thinking of turning it over, too so they can strip the insides of the walls, too.