A good place to start with is physical security. Install your FortiGate in a secure location, such as a locked room or one with restricted access. A restricted location prevents unauthorized users from getting physical access to the device.
If unauthorized users have physical access, they can disrupt your entire network by disconnecting your FortiGate (either by accident or on purpose). They could also connect a console cable and attempt to log into the CLI. Also, when a FortiGate unit reboots, a person with physical access can interrupt the boot process and install different firmware.