A type of rule that forces a player to do something they wouldn't otherwise do.
Considered a heavy-handed design choice, most designers try to avoid iron maiden rules as they take away player agency.
Examples of when an iron maiden rule might be used (and has been in some games):
France 1940. The French Army was recognized as the best in the world, including having world class tanks. And yet the Germans overran them in a few months. There are many reasons for this, but paramount was the French drive into Belgium while at the same time not really preparing for a German drive through the Ardennes. This allowed the Germans to swing around the rear of the Allied forces that had gone into Belgium and cut them off. In a game situation, any Allied player would be wise to not drive into Belgium and to be aware of the threat of the Germans coming through the heavily forrested Ardennes. To keep things moving historically, some designers have used iron maiden rules to force the attack on Belgium.
US Civil War, General Mclellan. Little Mac (as he was called by his men), was a superb administrator and trainer of troops. But he was incredibly timid when attacking. Always wary of losing troops (sometimes for political reasons), and always overestimating the enemy in front of him. So there were engagements where he had overwhelming force, but declined to attack. In a game situation, any player would attack in those situations. So the designer has to come up with a way to dis-incentivise Mac from attacking, or resort to some sort of iron maiden rule.
NB- not to be confused with the crappy band from the 80s that sold a million t-shirts.
