Most organizations discover their security gaps in the worst possible way -- after something goes wrong. A misconfigured firewall, an overly permissive access control, a flat network where lateral movement is trivially easy, a VPN that was never properly segmented from production systems. These are not exotic problems. They are common ones, and they are all architecture problems.
I have designed network infrastructure for healthcare systems where a misconfigured access control had direct patient safety implications. I have built environments that passed federal IL5 security audits and PCI-DSS assessments on the first submission. That is not because I got lucky. It is because security was designed in from the beginning -- not layered on at the end when someone asked if the environment was secure.
I have deployed and managed Cisco and Juniper infrastructure in enterprise environments. Routing, switching, firewall configuration, access control lists, network segmentation, VPN design, QoS, and the monitoring that tells you when something needs attention before it becomes an outage or an incident.