The next time you are traveling through time and find yourself in a medieval castle during a siege, you may need this list.
1. Water for drinking (by humans and any animals), cooking, and bathing
2. Food, either preserved and stored or as living plants and animals
3. Strong, tall walls
4. Defensive weapons to keep besiegers from climbing the walls
5. Wood or fuel/energy in some form, medicine, and spare parts for all contraptions