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.
Rules: Medieval
Castle Cracker Medieval and Ancient Warfare Rules
Castle Cracker is a set of free wargames rules for backyard games in the ancient and medieval eras.
Knights and Knaves Medieval Skirmish Rules
Once available as a commercial rules set — and a nice one it was — Knights and Knaves now is available as a download. Also available are the More Knights & Knaves and Castles, Cogs & Chevauches expansions.
Medieval Siege Rules
Ryan Scow offers a set of free medieval siege rules.
Agincourt Board Game
GMT Games offers a free download of a Battle of Agincourt game. If you’re familiar with their commercial board war games, you’ll know that they have wonderful graphics and gameplay, both of which are on display here.