The Mysteries of Womanhood
Coleridge had a rather complicated relationship with women. As is befitting the time, Coleridge believed that women were the inferior sex, and without the guidance of a man they would surely go astray. We see that in "Christabel." The title character is mostly left to her own devices, since her mother is dead and her father is said to be of poor health. Since she is left alone to make decisions about whether to let a strange woman into the house or not, it is only natural that she would make such a poor decision. Since women are so weak of spirit, it is also only natural that Christabel would fall under Geraldine's spell so easily.
On the flip side, and further demonstrating Coleridge's complicated feelings about women, Sir Leoline is also easily overcome by Geraldine's evil. Basically, even a big strong man can't resist a hot woman, especially when that big strong man is actually very small and broken inside because his true love died rather tragically by giving birth to a girl (who would be unable to become his heir because of feudal laws of the time). Complicated, contradictory, and a tad depressing, you say? It sure sounds a lot like Coleridge.
And if you're feeling short-changed by the abrupt ending of "Christabel," we recommend checking out "This Lime-Tree Bower My Prison" and "Dejection: An Ode."