Of course! We have one up in our downstairs now. It's pretty much a permanent fixture actually, the girls sleep there more than they do in their own room. The one up now is pretty elaborate. They've colonized our big 8-person camping tent, dragged a queen mattress down 2 flights of stairs from their bed, that's under the blanket tent, the top of which is attached to the sofa and extends to drape over the TV and games system. The younger kids' play kitchen holds up the other part of it, they've got all the extra blankets tied together with knitting yarn.
I would have died and gone to heaven if I was allowed to make a fort at all, let alone one like that, so I don't rain on their parade. I help and add on to it when I see a way, LOL! We can fit our entire family + kids' friends in there. This thing is pretty extended.
We're working on rebuilding their treehouse my husband made them at our old house, we took it apart when we moved and they're itching to have it back again, I'm sure if we get it finished this weekend, they'll be cramming as many overnights in there as they can before it turns too cold.