There's no denying that Coleridge sprinkled lots of spooky stuff into the poem, but very little of it is random or solely atmospheric.
The first part in particular is packed with very specific symbols from pagan religions and other mystical traditions. Coleridge lived in a time with a stronger sense of superstition than we have now, so many of these symbols would have been a lot more obvious to his audience. However, some of these are classics and we can pick them out easily even today. Most of these appear in the first half of the poem, before Geraldine is able to work her spells inside the castle, which helps us understand who (or what) Geraldine might be underneath all that pretty illusion.
Line 29: The forest is a sacred place in many pagan religions, especially in Druidism. Druids really dig trees. In fact, they believe that the forest is a temple. Now, why would Christabel, a character who appears to be Catholic in the story, choose to pray in a manner more befitting a Druid? It seems that this is Coleridge's way of exposing Christabel's innocence and naïveté early on in the poem. In her effort to not disturb her father, who does not sleep well, she goes outside to pray instead. Unfortunately, though she intends to engage in a pious and Christian act, she inadvertently engages in a pagan one instead because she doesn't know any better. Worse yet, by doing this (inadvertently or not), she makes herself even more vulnerable than usual to the (pagan) evils that lurk in the woods.
Lines 33-34: These lines specifically tell us that the tree Christabel is praying under is oak and that there is mistletoe on it. No big deal, right? Well, actually it is kind of a big deal for Druids, who believed that both the oak and mistletoe were sacred. Mistletoe, being a parasitic plant, appeared to grow without any roots or other connection to the earth. If a plant doesn't have a connection to the ground, then, logically, it must have connections to the heavens instead, making mistletoe a highly sacred plant. They also believed that these plants took on the qualities of the plants they grew on, and so the mixture of the holy oak and the sacred mistletoe together makes for a really powerful place. It's important to note that pagans would feel that this area would be quite safe, but Coleridge paints a picture of just the opposite, which tells us what he thinks of the pagan belief system.
Line 35: Coleridge is not being subtle here. He wants us to remember that this is not just any tree, but an oak tree, complete with all its pagan and spooky symbolism.
Line 42: Now it's not just an old oak tree, it's a "huge, broad-breasted" one. This will not be the first time Coleridge mentions breasts in this poem. In fact, this may be a bit of foreshadowing of what's to come later on in Christabel's bedroom.
Line 63: Geraldine's bare feet connect her directly to the earth. We're pretty sure hip Romantics referred to feet as "Druid sandals." Okay, maybe not, but it sounds good.