Site Reliability Engineer

Castelion is moving incredibly fast to develop and deliver advanced defense systems at a time when execution matters more than ever. We believe focus, ownership, and excellence are decisive advantages - and we're building a world-class team to turn bold ideas into real capability.

This is a rare opportunity to join at an early stage, where you'll have significant ownership, collaborate with exceptional teammates, and make a direct, measurable impact on our mission and the future of the company - regardless of your function.

We are seeking a Site Reliability Engineer to own the reliability, performance, observability, and operational health of Castelion's critical engineering systems. These systems support software development, CI/CD, artifact distribution, test infrastructure, developer workflows, and other services that engineers depend on to deliver hardware and software.

This role is the missing reliability piece of an existing high-performing engineering organization. You will work across Dev Ops, Cloud, Software, Security, Test, and IT to identify reliability risks, diagnose failures that cross system boundaries, and drive corrective actions to resolution. You will be expected to understand and improve existing systems rather than defaulting to replacement, using new technology when it solves a demonstrated reliability, scalability, or operational problem.

• Establish meaningful reliability, availability, latency, capacity, and recovery expectations for critical engineering services, with measurable health indicators and useful alerts.
• Lead deep technical investigations and incident response across application, Linux, networking, storage, Kubernetes, cloud, and other system boundaries; collect evidence, separate symptoms from root causes, and drive incidents through resolution.
• Build and improve monitoring and diagnostic systems that detect problems before users report them and provide engineers with the information needed to quickly understand and resolve failures.
• Analyze system performance and capacity across compute, memory, storage, networking, connections, and other constrained resources; identify operating limits and address issues through the simplest effective solution, whether optimization, additional capacity, scaling, caching, configuration changes, or architectural improvements.
• Drive evidence-backed root cause analysis and postmortem actions for significant incidents, ensuring corrective and preventive actions are implemented and verified to reduce recurring failures.
• Partner with Dev Ops, Cloud, Software, Security, Test, and IT to resolve reliability problems that cross team boundaries, providing technical leadership without attempting to own every component involved.
• Understand, operate, and incrementally improve systems built by other engineers, balancing reliability and operational value against existing architecture, constraints, and engineering practices.
• Participate in the on-call rotation for critical engineering services, providing first-response triage, escalation, and follow-up for recurring reliability issues.

• Bachelor's, Master's, or PhD in Computer Science, Computer Engineering, or a related technical field.
• 5+ years of experience in Site Reliability Engineering, Production Engineering, Systems Engineering, Infrastructure Engineering, or a related discipline supporting production or mission-critical systems.
• Demonstrated experience debugging complex production failures across multiple system layers and driving investigations from the first symptom to an evidence-backed root cause and lasting corrective action.
• Strong Linux systems expertise, including CPU, memory, storage, networking, processes, sockets, and system services, with a strong understanding of performance and capacity concepts such as IOPS, throughput, latency, queue depth, and connection concurrency.
• Experience building and operating observability, monitoring, alerting, and incident response systems, with the ability to distinguish between mitigation, workaround, corrective action, and preventive action.
• Strong networking and application fundamentals, including TCP, TLS, HTTP, DNS, reverse proxies, load balancers, connection states, and timeouts; able to investigate application runtime behavior such as threads, connection pools, file descriptors, memory, or garbage collection when the evidence points there.
• Demonstrated ability to work effectively within existing systems and…