The word unheeded is an adverb meaning without being noticed, disturbed, observed. It can go at the beginning of the sentence; it can go after the verb or it can go at the end of the sentence:
...the pleasant region where unheeded (adverb) I could commune with the creatures of my fancy.
- ...where I could commune unheeded with the creatures of my fancy.
- ...where I could commune with creatures of my fancy unheeded.
I felt that blank incapability of invention which is the greatest misery of authorship, when dull (adjective) Nothing (Subject) replies to our anxious invocations. The word dull is an adjective. It modifies the subject Nothing, a personified noun. (See Personified | Define Personified at Dictionary.com)