Junior General has a simple set of Medieval Castle siege rules. The site also has links to appropriate paper figures, castle models and siege equipment.
Siege
Spanish Fury Siege
From the Perfect Captain comes Spanish Fury: Siege, a board game about besieging a 16th century town or fort.
Medieval Siege Rules
Ryan Scow offers a set of free medieval siege rules.
Siege of Tenochtitlan rules
Matt Fritz has these simple set of free wargamesrules for recreating the Siege of Tenochtitlan. He uses them with his students — a great idea for both wargaming and learning! It inspires me to try similar things in my classroom.