Why did castles have moats?

Answer: Stop attackers from reaching walls

Provide water for the castle — Wrong. While moats contained water, their purpose was defensive, not supply. Moats prevented attackers from reaching castle walls with ladders, siege towers, or tunneling underneath. Castles had wells or cisterns for drinking water.

Stop attackers from reaching walls ✓ — Correct! Moats prevented attackers from bringing siege towers or ladders to the walls, and made tunneling under the walls (undermining) extremely difficult or impossible. The water barrier forced attackers to use slower methods (catapults, starvation), giving defenders more time. Some moats were dry ditches serving the same purpose.

Raise fish for food supply — Wrong. While moats occasionally provided fish, their primary purpose was defensive. The wide water barrier stopped siege equipment and prevented tunneling, protecting the castle walls from direct assault.