Defense-in-depth is an architecture where multiple layers of security controls are installed throughout an information and information systems of an enterprise.
Security controls can be technical, physical and administrative types. Deploying these security controls in layered fashion makes it harder for the attackers to break our security arsenals. Defense-in-depth is a security strategy in which several security layers are placed in information systems in a series arrangement. It is one of the most crucial principles of information security.
The goal of defense-in-depth is to prevent direct attacks against our information systems. We deploy endpoint security controls, firewalls, physical securities, access controls, encryption and so on controls in a layered fashion to stop intruders.