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C4 Model Case Study – Supplementary Diagram: System Landscape Diagram

Purpose of the System Landscape Diagram (C4 Level 0 / Landscape)

The System Landscape diagram is the widest view in the C4 model family.
It sits above the System Context diagram and shows:

  • All major software systems that are important from the perspective of the organisation / business unit / product line / team

  • How those systems interact at a very high level

  • Which systems are internal, external, legacy, strategic, being replaced, being built, etc.

  • Often includes people / actor groups only when they are central to multiple systems

It answers questions such as:

  • “What does our entire digital estate look like today?”

  • “Which systems talk to our core banking mainframe?”

  • “Where are the biggest integration pain points?”

  • “Which new initiatives are touching many existing systems?”

It is not focused on one single software system — it is the organisation-wide / department-wide big picture.

Good Titles for a System Landscape Diagram

Choose a title that clearly communicates the scope and perspective. Some strong, commonly used examples:

  1. Big Bank plc – Enterprise / Core Banking Landscape 2025

  2. Personal & Small Business Banking – System Landscape

  3. Retail Banking Division – Major Systems & Integrations

  4. Internet Banking Programme – Surrounding Ecosystem (2025)

  5. End-to-End Customer Journey – Banking Systems Landscape

  6. Strategic Systems Landscape – Big Bank plc (focus on Customer-Facing Channels)

  7. Current State Architecture – Retail & Digital Banking Portfolio

Recommended title for this case study (broad but focused):

Big Bank plc – Retail & Digital Banking System Landscape (2025 Target State)

When to Create / Update a Landscape Diagram

  • During early discovery / architecture runway phase

  • When doing portfolio rationalization or legacy modernisation planning

  • As part of quarterly / annual architecture review

  • When onboarding senior leaders or regulators who need the 30,000 ft view

  • Before starting a large programme that touches many systems

PlantUML / C4-PlantUML Example – System Landscape DiagraC4 Model Case Study – Supplementary Diagram: System Landscape Diagram

@startuml
!include https://static.visual-paradigm.com/plantuml-stdlib/C4-PlantUML/master/C4_Context.puml

LAYOUT_TOP_DOWN()
LAYOUT_WITH_LEGEND()

title System Landscape diagram for Big Bank plc

Person(customer, “Personal Banking Customer”, “A customer of the bank, with personal bank accounts.”)

Enterprise_Boundary(c0, “Big Bank plc”) {
System(banking_system, “Internet Banking System”, “Allows customers to view information about their bank accounts, and make payments.”)

System_Ext(atm, “ATM”, “Allows customers to withdraw cash.”)
System_Ext(mail_system, “E-mail system”, “The internal Microsoft Exchange e-mail system.”)

System_Ext(mainframe, “Mainframe Banking System”, “Stores all of the core banking information about customers, accounts, transactions, etc.”)

Person_Ext(customer_service, “Customer Service Staff”, “Customer service staff within the bank.”)
Person_Ext(back_office, “Back Office Staff”, “Administration and support staff within the bank.”)
}

Rel_Neighbor(customer, banking_system, “Uses”)
Rel_R(customer, atm, “Withdraws cash using”)
Rel_Back(customer, mail_system, “Sends e-mails to”)

Rel_R(customer, customer_service, “Asks questions to”, “Telephone”)

Rel_D(banking_system, mail_system, “Sends e-mail using”)
Rel_R(atm, mainframe, “Uses”)
Rel_R(banking_system, mainframe, “Uses”)
Rel_D(customer_service, mainframe, “Uses”)
Rel_U(back_office, mainframe, “Uses”)

Lay_D(atm, banking_system)

Lay_D(atm, customer)
Lay_U(mail_system, customer)
@enduml

Quick Interpretation Guide

  • Left side — mostly new/cloud/modern initiatives

  • Centre / right — legacy core and heavyweight integration hubs

  • External systems marked with _Ext

  • Legend automatically generated thanks to LAYOUT_WITH_LEGEND()

  • Boundaries used only where helpful (here around the new digital channels)

This diagram type is intentionally coarse-grained — it usually does not show containers or components, databases, protocols, etc. Those details belong in System Context and Container diagrams of the individual systems.

Would you like to:

  • Extend this into a current state vs target state comparison?

  • Add colour coding (legacy = orange, strategic = green, etc.)?

  • Focus on a smaller scope (e.g. only payments landscape)?

  • Convert this example to Mermaid instead?

Let me know how you’d like to adapt it.

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