Interior Castle is one of the most advanced books on prayer ever written. Of course I want you to read it yourself, but it's pretty profound, so here's a bit of a road map: It goes through the seven different "conditions" in which the soul can be in this life, which are called "mansions" of the interior castle of the soul. The interior castle is made of diamond with the seven mansions in a spiral. God is in the seventh mansion in the center. His light radiates throughout the mansions through the diamond. The soul moves from one mansion to the next through the maturation of her prayer life. She can easily move backwards by committing sin or being lazy in the faith. It is much easier to move backwards than forwards, so it's like the spiral is arranged as a cone, the path inwards going up, with the seventh mansion at the top.
When a soul enters the castle, there are demonic creatures attached to her, such that she is blinded by them and cannot see. The creatures try to get her to sin and be slothful, thus moving backwards. God's light on the other hand, pulls her further in. Because of God's light, the demons can only survive in the outer mansions.
The first mansion is an existence of frequent mortal sin. Demons have a huge presence and largely dominate the mansion. There is very little prayer, and what little prayer exists, is the way out into the next mansion.
The second mansion is marked by occasional mortal sin and frequent venial sin. The soul begins to pray regularly. It is a tug of war over the soul with both God's light and demons holding large amounts of influence over the soul.
The third mansion is so close to God (by comparison), that the demons' influence is so limited, that the soul no longer commits mortal sin. Such sins disgust her, although she commits occasional venial sins. This mansion is marked by aridity and dryness in prayer. God gives the soul difficulties in prayer so that the soul might overcome them, become stronger, and advance further.
In the fourth mansion, she is so far in, demons cannot survive for more that very brief periods. They are like fish out of water that suffocate on God's awesomeness. The only attack that still works is distortions of the faith, aimed at making the practice of the faith more relaxed and easy-going so the soul might fall into the demon's reach. This mansion is marked by experiencing great pleasures in prayer. It is the beginning of contemplative prayer. Pleasures come in two ways: the soul's own attempts to please herself in prayer and God's attempts to please her in prayer. Pleasures from God are much greater, so she must not try to force pleasures on her own, which causes dryness, but let God give them when He wishes.
The fifth mansion is completely free from the presence of demons. It is too close to God for them to survive.
The soul is completely free of their sinful snares, but is never secure in this freedom. The demons can only call out to the soul to distract or discourage her, so she can always fall back. The fifth mansion overlooks the first mansion in this spiral and the soul can see those sinners in it (who reject her for her sanctity). In this mansion the soul profoundly submits to God's will, which brings both sorrow and joy. She sees the world, it's sin, and it's sinners as God sees it. She sees their sins and is greatly offended by all of them, even the smallest sins, simply because they offend God. Things of the world, even those that are not sinful, bring her no pleasure anymore, they are no longer fun. Only God brings pleasure, only God is fun anymore. She feels sorry for the sinners and worries about their salvation. She cares about them as if they were her own children. This conformity to God's will also is the beginning of a profound union with Him, which causes great pleasure. She desires more union at this profound level, but that desire is not yet satisfied, so there is another kind of dryness.
The sixth mansion overlooks the second and the less holy souls therein (lukewarm and cafeteria Catholics) reject her for her sanctity, accusing her of being arrogant in her practice of the faith, preferring their minimalist approach. In this mansion, God profoundly reveals Himself to her in greater quantities as she desired, by way of speaking to her soul, or even her body (through audible sound) in private revelations or through rapture and ecstasies in prayer. St. Theresa goes on at length about this and warns her sister not to confuse figments of their imaginations for revelations. God increases the desire for profound union by a very large amount, but does not yet satisfy this more intense desire. This causes a great spell of dryness, since she has a great desire for even more union with God at this profound level, but isn't receiving it. This great depression is what St. John of the Cross calls "the dark night of the soul".
In the seventh mansion, aridity and suffering almost cease entirely. She finds a peace and ecstasy greater than before with less suffering than before. She forgets herself and focuses on God alone. She actually gains an even greater desire for suffering than before, since it unites her to God, although no amount of suffering decreases the peace and joy in her soul, due to that union with God. She can converse with the Trinity and this is the largest taste of Heaven she can get in this life. It's the closest she can get to seeing God face to face. She feels safer than before, as if the slope has leveled off and she knows she won't fall to outer mansions, although it's still possible.