Senior Web Developer

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.

Castelion is seeking a hands-on Senior Web Developer to lead the architecture, development, and production readiness of the Seeker Team’s internal web application. This role will unify existing tools for calibration, field testing, ML annotation and training, and Foxglove into a cohesive, intuitive product that engineers and analysts rely on throughout the day.

As the application’s technical owner, you will work directly with end users to understand their workflows, make large-scale data accessible and responsive, and build reliable infrastructure that supports full-rate hardware manufacturing. You will remain deeply involved in implementation while setting technical direction, prioritizing work, and mentoring junior developers on a small team.

Responsibilities

• Own architecture and development: Lead the web application from design through production, establishing a scalable architecture while remaining hands-on with implementation.
• Build a unified user experience: Integrate calibration, field testing, ML annotation, ML training, and Foxglove into a single application with clear navigation and consistent workflows.
• Partner with users: Work directly with engineers and analysts to understand their needs, simplify workflows, and reduce manual steps and one-off scripts.
• Optimize large-scale data access: Design effective fetching, caching, pagination, streaming, precomputation, background processing, and visualization strategies to support terabytes of data.
• Deliver a high-quality internal product: Improve usability, adoption, responsiveness, and reliability so users can find tools and data without developer assistance.
• Build for production at scale: Establish reliable infrastructure, monitoring, automated testing, and safe deployment practices, including frequent releases and straightforward rollbacks.
• Prepare for manufacturing scale: Ensure the application is ready to support full-rate hardware manufacturing by the end of 2027.
• Provide technical leadership: Set priorities, assign work, review designs and code, and mentor junior developers while contributing directly to development.
• Evolve existing systems thoughtfully: Work within Castelion’s current technology stack where practical, making changes or replacements based on demonstrated results rather than personal preference.

Basic Qualifications

• Experience leading a web application from architecture and implementation through production deployment.
• Strong proficiency in Python, FastAPI, and asynchronous programming withasyncio.
• Experience with PostgreSQL, object-relational mapping (ORM), and blob or object storage.
• Demonstrated ability to design responsive applications for large datasets and data-intensive workflows.
• Experience building reliable applications with automated testing, CI/CD, monitoring, and safe deployment and rollback strategies.
• Strong product judgment, user experience instincts, and the ability to translate complex workflows into simple, effective software.
• Ability to mentor and delegate to junior developers while remaining hands-on.
• Demonstrated ownership of application quality, usability, reliability, and user adoption.

Preferred Skills and Experience

• Experience with Plotly or similar data…