What is the castle and moat security model?

Castle and moat is a security model in which no one outside a network can access an organization's data, keeping external threats out. In this model, the “castle” represents the organization's internal system or network that contains sensitive data, while the “moat” represents the perimeter defense that blocks external threats from entering. The idea is that no one outside the moat can get inside the castle, but once inside, there are fewer security controls. This model focuses more on external firewalls and defenses, whereas a zero-trust security approach rejects the castle and moat model because it requires all users and devices to be continuously and explicitly validated when accessing a network, system or data.

Castle and moat vs zero trust: What's the difference?

The zero-trust framework is a security model that eliminates implicit trust, assuming that security risks can originate from both inside and outside the network. This model requires every user and device to explicitly verify their identity before being granted access to any data, regardless of their location. On the other hand, the castle and moat structure automatically assumes that users inside the network can be trusted.

While both security models are commonly used, the zero-trust model offers a more secure approach because cyber attacks have become more advanced, and the perimeter of organizational networks has become less distinct.

How the castle and moat model works

The castle and moat model operates on three primary principles.

Strong perimeter defenses (the "moat"): This involves security measures at the network boundary, such as firewalls, intrusion detection systems and strict access controls for external connections.
Relatively weaker internal security controls (inside the "castle"): This means that once inside the network, there are fewer security checkpoints and barriers between different internal resources compared to the strict perimeter controls. Users still need appropriate permissions, but the internal security architecture is less stringent than the perimeter defense.
A clear distinction between "inside" and "outside" the network: This means treating internal and external traffic fundamentally differently. Internal traffic is considered more trustworthy by default, while external traffic faces strict scrutiny.

Problems with the castle and moat model

Here are some of the biggest challenges associated with the castle and moat security model.

The model is outdated

One of the major weaknesses of this model is its reliance on a single perimeter to protect the entire network. As cyber threats become more complex, cybercriminals can more easily bypass a traditional perimeter through social engineering tactics, credential theft, targeted malware attacks and exploitation of security vulnerabilities. Once cybercriminals breach the moat, they can move laterally within the network, accessing sensitive data. This dependence on a single layer of defense makes the perimeter less relevant and increasingly insecure.

Cloud makes it difficult to implement

Cloud computing complicates the castle and moat model because it eliminates the concept of a fixed perimeter. In a cloud environment, organizations store data and resources across multiple locations, and users can access them from anywhere, whether inside or outside the organization's physical network. With resources distributed across cloud platforms and accessed remotely, securing a fixed boundary becomes ineffective, making it challenging to fully protect an organization's data.

Susceptible to credential theft

Cybercriminals can steal login credentials through methods like phishing or exploiting data breaches, bypassing the perimeter defense. If they manage to compromise an authorized account, they gain access to the network and technically appear “trusted” since they are inside the network. This allows cybercriminals to access sensitive information without raising alerts, making malicious activities harder to detect.

Vulnerable to cyber attacks

The castle and moat model mainly focuses on blocking external threats, which means it overlooks vulnerabilities within the network. For example, it fails to protect organizations against insider threats, where trusted, authorized users may accidentally or intentionally compromise security. Since these insiders are already within the network, they don't need to bypass the perimeter defenses.

Requires constant monitoring

Since threats can emerge from both inside and outside the network, constant monitoring is necessary to detect any suspicious activity. While the moat may help keep most external threats out, there's always a risk that an attacker could find a way to sneak inside the castle. Once inside, they could then access sensitive data and cause harm. Without continuous monitoring, organizations may miss signs of a security compromise.