Playpens in family bedrooms for those under about 18 months and/or doing two naps. Older ones on mats in our diningroom which is our mainfloor playroom. Gradually I will transition the toddlers to the main naproom by bringing their playpen down and have them get used to being in the room but sleeping with the others. When I am happy that they are understand the concept of naptime I move them to a mat as an earned privilege putting them back into the playpen if they won't stay on the mat - since there are enticing toys around they have to be mentally able to understand the concept of stay put.
I don't use the basement for naps as I like to be on the mainfloor during nap and need to close and lock the door to the basement for my own daughter (special needs) who is not allowed on stairs alone but of course doesn't nap.