The poem "One need not be a chamber to be haunted" deals with a contrast between fears caused by external phenomenon and those caused by our own brain. We see this contrast in the first stanza when the speaker maintains that
The brain has corridors surpassing
In other words, what is inside our brain is much more frightening than any tangible thing or place. This idea is further developed in the second stanza in with the "interior" is more dangerous than an "external ghost." The speaker declares that we can arm ourselves against the "assassin, hid in our apartment," but cannot defend ourselves against the intruders of our minds.
Dickinson is vague as to what we have within that is so frightening. But it could be any number of things: regret, guilt, imagination, anxiety, paranoia, fears of loss or death--anything that makes us fearful to continue on with life or reluctant to get out of bed.