In "Moria" (a classic computerized fantasy-role-playing game) there's a magical spell called Glyph of Warding. When cast, this enchantment puts a symbol on the dungeon floor which monsters can't cross over. It's a purely defensive bit of trickery that only works in a narrow corridor; eventually the glyph can be broken by a persistent attack. But in the meantime such a barrier may give a hard-pressed adventurer a chance to retreat, recover from injuries, or in some cases launch a counterattack using a distance weapon.
The Glyph of Warding concept --- a sign that says Stop! and really means it --- appeals to me ... and not just because of the rampant tendency around here of drivers to roll through ordinary stop signs. It's another display of the magic of language, as alluded to in KeyToTheTreasure (23 Apr 2004). I thought of it again recently when I saw an article in The Numismatist titled "They Shall Not Pass: A Medal for Verdun" by Marilyn Reback (November 2003). In French the medallion quotes the words of General Petain: "Ils ne passeront pas". Halt!