The subject invention features an entrance module for a height adjustable enclosure with a top portion movable with the respect to a bottom portion. The preferred entrance module comprises a hingedly attached platform movable between a stowed folded position and a deployed unfolded position. A locking assembly releasably locks the platform in the folded position independent of the position of the top movable part of the shelter. The preferred locking assembly includes a rail attached to the top movable portion of the shelter, one of a female and a male portion of a lock attached to and slideable along the rail, and the other of the female and the male portion of the lock attached to the platform.