Where there's like prison cells etc?  You have to arrange the
turrets, then survive an onslaught of soldiers for a period of time
til Alex comes... I mean arrives... :-)

Enemy soldiers are on their way to find you. The turrets (the stand
alone guns) will shoot at the soldiers. Pick them up and put them down
anywhere you like (you can move them again if you want) so that you can
defend yourself.

If you still have problems, post back here.

(Note to others - try not to post any spoilers yet. Let's wait for him
to come back here before we hand out any more hints.)

Crawl through hole in the far wall.
                                                      Doug