If you have a platform taller than 4', you need to put railings on it that will keep actors from falling to their deaths. If you have an audience platform taller than (some smaller number that I'm not sure of) you'll also have to create railings for them.
Some notes about good railing construction:
-You probably want to make the railings 3' to 3.5' off the platform surface. Much taller starts blocking people's faces, much shorter becomes a bit worrying.
-It's surprisingly easy to make stable railings. Don't worry too much about adding lots and lots of wood. The Othello railings had about 4' gaps between each vertical support, and they were fine.
-Railings don't need to be 2x4s. Othello used 2x3s to good effect. Twelfth Night used 1x3s, which seemed fine (though weren't intended to be structural)
-If your railings need to be more than 8 feet long, consider buying wood that's long enough to span that whole length, since it will be easier to make it structural.
-The more places your railings are connected to something at right angles to them, the more structural they will be. Opportunities for this include corners of the platform, staircases with their own railings, flats, and so on.
-Try to make sure actors don't get splinters
-Making railings a bit longer and letting them continue further down along the platform can make them more structural.
A picture showing one reasonable way to construct railings:
A totally different take on railings:
Flats make good railings. They're even more structural than normal railings sometimes! Plywood can also make good railings. Just align the plywood with the side of the platform and screw it in. Maybe add a few 2x4s screwed into the plywood to make it a bit less wobbly. Run lines of gaff tape along the seam between flats or plywood to get it to look like one solid piece.