Critical hits in Lexandria are handled the way TSR suggests in Players Option: Combat and Tactics with two exceptions.

1) A character or monster can not cause a critical hit that is higher in severity (the die roll) higher than his/her class or hit dice.

2) A character or monster causes a critical hit on a natural roll of twenty. A weapon grandmaster causes a critical hit on a natural 19 or 20.

This will prevent weak characters or monsters from causing unbalancing effects on game play.