The whole poem—what there is of it—takes place either inside Sir Leoline's castle or in the wooded area just outside it. It all seems pretty straightforward—but we certainly know by now that straightforward is not how Coleridge works.
The castle is an orderly place where watchdogs keep time with the clocks and everyone observes the right prayers and customs at exactly the right time. The forest outside, however, is a wild, chaotic, and unruly place where mysterious and frightening things happen. Inside the castle, Christabel maintains her innocence, but that innocence is threatened the minute she ventures outside. Just like the annoyingly naïve girl in a horror movie, Christabel is dumb enough to invite evil inside, allowing it to threaten the innocence and peace of everyone in the castle. Christabel's is the trusting and silly weakness that allows evil to penetrate what was once the perfect fortress against such darkness.
That isn't to say that there aren't any safety measures inside the castle. Signs of Leoline's power and prowess on the battlefield decorate the hallways. Plus, Christabel's bedroom seems to resemble a Victorian version of a Hot Topic display, what with all its demons and angels and other Gothic decorations. All of this is meant to scare anything with bad intentions away if it happens to get inside the castle. And sure, it slows Geraldine down a bit, but—unfortunately for everyone in the castle—it doesn't really stop her from her sinister plans.