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.
medieval
D3 Norman Conquest Rules
The D3 Norman Conquest rules are designed as a set of low-complexity rules based on One Hour Wargaming rules and Dux Bellorum. It uses a square gridded tabletop.
Early Blazon Heraldry Site
Early Blazon offers illustrations of hundreds of coats of arms through the Crusades.
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.