Being trapped in a dream usually reflects a feeling of helplessness in some part of your waking life -- a job, a relationship, or a routine where the exits don't seem to exist even though logically you know they must. The specific place you're trapped in often mirrors exactly where that stuck feeling is coming from.
To unfasten the tongues Mercury must be in its house and it is efficient with those who Mercury rules. - Picatrix (Andalusia, ~1000.AD)