Commentary on Meditations: B8:48 - The Inner Citadel

Remember that your directing mind becomes invincible when it withdraws into its own self-sufficiency, not doing anything it does not wish to do, even if its position is unreasonable. How much more, then, when the judgement it forms is reasoned and deliberate? That is why a mind free from passions is a fortress: people have no stronger place of retreat, and someone taking refuge here is then impregnable. Anyone who has not seen this is short of wisdom: anyone who has seen it and does not take refuge is short of fortune.

Within the refuge of your Inner Citadel, you are completely yourself.  No one can tell you what to think or how to act.  When you realize that the true, unique version of "you" lives in a mental fortress, you become free to act and reason.  No longer are you pulled this way and that by external events and people.  The only way to get to this realization is to constantly apply the discipline of assent.  The is a matter of creating gaps between your mental fortress and external events - constantly.  By this continuous exercise, all passions and events "lose power" over you (or perhaps better said, you realize that you were granting these things too much power in your mind).  And furthermore, once you've realized you have a mental fortress, you are free to apply reasoned and deliberate judgement of everything.  And furthermore, if you focus on attaining virtue, you are on your way to wisdom.

(see also Citadel p. 120, 122 [read that entire page])