According to the
OED
, a "bower" most often refers to a dwelling, or a cottage. However, an additional definition that refers to poetic use of the word is interesting when considered in the context of
This Lime Tree Bower My Prison. In this definition a bower is a poetic word for an
idealized abode.
In Coleridge's poem however, his bower is not an ideal place, but instead a confining space from which he yearns to break free. To have bower, a typically ideal place, become a prison adds emphasis to Coleridge's desire to join his friends in "wandering in gladness" out of doors with the now idealized natural world.