A castle did not rely on a single wall. It had a moat, a wall, a gate, guards and a keep. If the moat failed, the wall was next. Security uses the same idea, called defence in depth: several independent layers, so that when one fails, another still stands.

Here are six layers a school or a small company might use:

• People: awareness, clear rules, someone to ask.
• Perimeter: the edge between the outside and the inside, such as a firewall and locked doors.
• Network: how the inside is divided and watched.
• Devices: updates, screen locks, antivirus, disk encryption.
• Applications: sign-in checks, safe settings, up-to-date software.
• Data: backups, encryption, and rules about who can open what.