When you immure someone or something, you put it behind a wall, as in a jail or some other kind of confining space.
You may recognize the -mur- in immure as the root for "wall," as in mural, which is a painting on a wall, or intramural, literally "inside the walls," as, for instance, the walls of a school — intramural sports are played among teams from the same school. You don't need a jail to immure someone. Rapunzel was immured in her tower. At the end of Shakespeare's Romeo and Juliet, the lovers are immured in the tomb.