Force fields have been around since Fm created them in Seige of Nor'Korh, but they've been one-sided (That is, they work from one side, but are passable from the other).

Then Jeff Swartz created dual sided force fields in an award winning map for the Evil Map Making Contest called "FireCracker".

Here, by Michael Watson, is a description (and demo map) showing how to make two-sided forcefields. Nicely done.