> For the complete documentation index, see [llms.txt](https://docs.defguard.net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.defguard.net/1.6/in-depth.md).

# In depth

- [Secure By Design](https://docs.defguard.net/1.6/in-depth/secure-by-design.md)
- [Architecture](https://docs.defguard.net/1.6/in-depth/architecture.md)
- [How do VPN statistics work](https://docs.defguard.net/1.6/in-depth/architecture/how-do-vpn-statistics-work.md)
- [Security concepts](https://docs.defguard.net/1.6/in-depth/architecture/security-concepts.md)
- [MFA Architecture](https://docs.defguard.net/1.6/in-depth/architecture/architecture.md)
- [Architecture Decision Records](https://docs.defguard.net/1.6/in-depth/architecture-decision-records.md): What are Defguard's Architecture Decision Records?
- [1.6](https://docs.defguard.net/1.6/in-depth/architecture-decision-records/1.6.md)
- [1.5](https://docs.defguard.net/1.6/in-depth/architecture-decision-records/1.5.md)
- [1.4](https://docs.defguard.net/1.6/in-depth/architecture-decision-records/1.4.md)
- [1.3](https://docs.defguard.net/1.6/in-depth/architecture-decision-records/1.3.md)
- [Pre-1.3](https://docs.defguard.net/1.6/in-depth/architecture-decision-records/pre-1.3.md)
- [Roadmap](https://docs.defguard.net/1.6/in-depth/roadmap.md): This is a high-level project roadmap of planned features.
- [Release cycle](https://docs.defguard.net/1.6/in-depth/release-cycle.md)
- [Client application feature compatibility](https://docs.defguard.net/1.6/in-depth/client-application-feature-compatibility.md)