Wizard Bob has created himself a mighty tower, but being a reclusive, paranoid crazy sort of Wizard he'd like to create it entirely sealed to stop anything getting in that he doesn't want inside.
The problem is that he'll rapidly run out of breathable air inside that tower and that's a problem as Bob likes to be able to breathe.
The air bubble spell creates a short term (1 min/lvl) pocket of air for breathing air, but that's no good for Bob's familiar or any creatures he summons and wants to chat with.
How can Bob ensure there is a fresh supply of air in his tower fortress without adding any holes in the wall?
Items are sub-par unless they affect an area rather than a single person. Spells are only OK if they will still be working when Bob has been away for a month or three.