Jul 24, 2025

By Rob Forbes

Zero Trust Security Architecture: Replacing the Old Perimeter Model

Discover how replacing perimeter defenses with a canal-based Zero Trust model gives you granular control over every access request while strengthening security through continuous verification and microperimeters.

Goodbye Moat-and-Castle, Hello Canal-Based Control

For decades, cybersecurity has relied on a mental model rooted in medieval defense: the castle and moat. Build high walls, guard the gates, and trust everything inside. It worked well enough when organizations had clearly defined perimeters, centralized infrastructure, and employees working on-site.

But today, those assumptions no longer hold. The perimeter is gone. Cloud services, remote work, APIs, third-party integrations, and machine identities have dissolved the walls—and the moat isn’t keeping anything out. In this new reality, Zero Trust has emerged not as a toolset, but as a strategic model for how access should be granted and governed.

To truly understand and implement Zero Trust, we need to move beyond the castle-and-moat analogy. A better metaphor for today’s dynamic environment is the canal.

The Zero Trust Canal: Controlled Flow Instead of Fortified Walls

In a Zero Trust world, access is no longer binary nor is it granted just based on location or implicit status. It’s earned, calculated, and continuously re-evaluated. A canal is a perfect analogy for how access should work: It’s a controlled journey akin to a ship navigating a canal system. The S.S. Access (an individual access request) must pass through a series of locks that verify trust conditions before reaching its destination: the protect surface (the resource).

A canal is a powerful metaphor for Zero Trust because:

  • It’s engineered for controlled movement
  • It uses locks to regulate and verify flow
  • It segments pathways to prevent uncontrolled spread

Just like a canal, Zero Trust is about deliberate, policy-driven movement where access request is guided through a series of checkpoints, each enforcing its own set of rules.

The Primary Locks of Zero Trust

Access requests must pass through a sequence of five primary locks. Each one is independently enforced and may contain multiple locking bolts, which are specific policy controls enforced through microperimeters.

  • Identity: Is the user, device, or service authenticated and authorized right now? Controls include biometrics, passkeys, roles, UEBA, and just-in-time access.
  • Network: Is the request coming from a trusted network segment? Enforced through segmentation, ZTNA, geo-filtering, and anomaly detection.
  • Device: Is the device secure and compliant? Checks include OS version, patches, encryption, and telemetry.
  • Application: Is the app or service behaving as expected and authorized for this interaction?
  • Data: What is being accessed, and what protections apply? Evaluates sensitivity, encryption, classification, and access policies.

Each lock can adapt in real time, triggering additional requirements such as multi-factor authentication (MFA), session isolation, reduced access scope, or full access revocation based on risk signals, user behavior, or policy changes.

Why This Model Works

AspectCastle & MoatCanal & Locks
PhilosophyKeep threats outContinuously verify trust at each stage
ArchitectureStatic perimeterDynamic, layered trust microperimeters
Trust AssumptionsEverything inside the castle is trustedTrust nothing by default
Blast RadiusBroad lateral movement if breachedContained through segmented microperimeters
Enforcement LocationPerimeter firewallInline, context-aware microperimeters
Decision DriversStatic rules and VPN accessReal-time context from Identity, network, device, app, and data pillars.

Microperimeters: Turning the Locks into Action

While the canal metaphor helps conceptualize the flow of access, microperimeters operationalize the locks. They are the enforcement boundaries that wrap around every sensitive asset or interaction.

Instead of relying on a single, monolithic perimeter, Zero Trust distributes enforcement to the edges of every access point. Whether it’s a device accessing a cloud resource, a microservice talking to another, or a user initiating an API call—each of these moments is surrounded by its own microperimeter, where context is evaluated and policy is applied.

These microperimeters allow for granular control and dynamic response. They enable organizations to move beyond static segmentation and build transaction-level access governance without relying on a rigid infrastructure model. In Zero Trust, trust is ephemeral, access is conditional, and enforcement is everywhere.

The Traditional Perimeter is Gone. Trust Must Flow.

The era of fixed perimeters is over. Today, every identity is potentially external. Every access decision is a potential breach point. And every application, API, and dataset is a potential target.


Zero Trust is not about blocking access. It’s about continuously granting access intentionally.


Build a system of canals, not walls. Raise locks, not gates. Let the S.S. Access travel securely—always evaluated, never assumed, never trusted.

To learn more about Zero Trust, check out our guidebook. For details on our capabilities / offerings, contact us.

Practical Guidance & Threat Intelligence

Related resources 

Stay a step ahead of the competition–and attackers–with fresh perspectives, practical guidance, and the latest threat intelligence. 

View all
Contact Us

Solve what’s next in cybersecurity  

Let’s talk about how we can support your next move toward a stronger, more secure digital foundation. 
Get in touch