"Magic exists to serve man and never to rule over him" - The Chant of LIght
The Chantry believes magic to be dangerous, as mages are the only ones who are consciously able to enter The Fade which makes them far more susceptible to demonic possession; therefore any child that demonstrates magical ability is sent to the Tower to be trained by the Circle of Magi and guarded by the Templars.
Apprentices are trained by senior mages in the ways of magic and taught about the dangers of blood magic. One of the most important lessons an apprentice learns is to be aware of the dangers of The Fade and how to avoid becoming possessed by a demon and becoming an abomination.
Mages are not allowed to leave the Tower without the First Enchanter's permission, which can only be given with reasonable cause. A phylactery of each apprentice's blood (the phylactery is sent to Circle headquarters in Val Royeaux, in Orlais when an apprentice becomes a full mage) is stored in the Tower's basement; the phylactery of blood is used by the Templars to track down escaped Circle mages (apostates). While the Templars imprison mages in the Circle, they also protect them from those outside who fear magic and are provoked to violence from their fear and hatred for mages.
The Templars who guard the Circle Tower are led by a Knight-Commander, there is often friction between the Knight Commander and the First Enchanter, who leads the mages. There is a constant power struggle in the Tower between the Templars and the mages (many of whom resent the Templars' control over them), this occasionally breaks out into rebellions led by groups of mages, although the Circle as a whole has never rebelled.