If, logically speaking, the player should be able to go back to an earlier
area, they should be able to do so. If you want to force the player into
another section, there are plenty of ways to make something one-way:
a ledge to jump off of that you cannot climb up again, a door that
closes and locks behind you, a river that carries you downstream to the
next section.
Try to make sure that there is no time where a player has to restore
a game to get back to an earlier point because there was an object they
missed they can no longer get. One of the easiest ways to do this is
to put an object needed for the current part and one needed for the
next part together so they must both be picked up at once.
-- Jason Dyer - jdyer@indirect.com