Eternal tower setting (please help)

[motoko's ghost]

Are there windows? If not, how does the air get in? A population large enough to have an armed guard that merely consists of "most of them" would use up air over several years, especially given the size of the castle. Is there plant life, that could convert carbon dioxide to oxygen, and does it require light or bioluminesce? Also, there needs to be some source of heat, as no matter the material it will get cold in winter. Is there a source of heat that doesn't use oxygen?

All these questions spring from one: Is it possible to look, but not actually go, outside the tower?

EDIT: Note 2 self: read the OP. I missed the fact that Greenhouses can provide ample plant life for both oxygen and food-related needs, as well as fuel for winter. However, Greenhouses work through light brought in from outside. Where's the light coming from, and would it be accessible to a suitably resourceful and foolhardy villager?

There is a lot of magic used(continual lights,etc) for the light purposes, the sheer bulk of the tower and its outer walls means that heat is lost very slowly, there's very little need for temperature regulation.
For the safety of the tower(or so its been said) any passageway of even view-way of the outside has been destroyed/collapsed/sealed behind 20ft of reinforced masonry.
...Except for the rooftop accessible only from the top floor and by the ruler of the city(which does not exist, according to the rulers, any such rumours are seditious in nature and will be punished.)

Ninjadeadbeard your idea of linked castles is even better than what I originally thought up,very nice and fantasy. However the tower is basically a single huge spire,nestled in a mountain range (and is itself practically a mountain). There are certain areas that seem to be larger than they should be, but the layout of those areas is too chaotic to really get accurate measurements(yes there are rooms bigger on the inside,doors that open to different rooms in different ways,etc left over from the original builders of the spire, however the age and extensive and repeated rebuilding of the currently populated areas means they are mostly in out of the way places, and may function chaoticly/intermittently)