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.
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.
Ryan Scow offers a set of free medieval siege rules.