What was a common medieval way to defend a castle with water?

The story behind the answer

Medieval castles often ringed themselves with a broad, water-filled ditch. This barrier slowed infantry, blocked siege towers and battering rams, and made underground sapping far harder; attackers were forced to crowd onto a narrow drawbridge under arrow and crossbow fire. The water could collapse mining tunnels and required besiegers to build awkward floating bridges or throw in fascines before any assault.

These defenses were usually fed by diverted streams or springs and were sometimes even stocked with fish; in low-lying regions they could be tidal. Not every site had enough water—some relied on dry ditches—while a watergate was just a controlled riverside entry, and a millpond powered mills rather than protecting walls. Myths about crocodiles are later inventions, but the stagnant stink could be very real.