Senior Web Developer

Why Castelion, Why Now

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.

Senior Web Developer

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 with asyncio.
• Experience with PostgreSQL, object-relational mapping (ORM), and blob or object storage.
• Strong experience with a modern frontend framework and JavaScript or TypeScript, HTML, and CSS.
• 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 visualization libraries.
• Experience building internal engineering tools, scientific applications, or data-intensive web platforms.
• Familiarity with large-scale data discovery, visualization, and retrieval patterns.
• Experience integrating multiple tools or workflows into a unified application.
• Experience with background jobs, precomputation, streaming data, and performance optimization.
• Experience preparing applications for high-availability or production-critical environments.
• Ability to balance architectural decisions with near-term delivery in a small, fast-moving engineering team.

Castelion offers a generous benefits package. Please refer to the bottom of our Careers page for more details.

Other Duties

Please note this job description is not designed to cover or contain a comprehensive listing of activities, duties or responsibilities that are required of the employee for this job. Duties, responsibilities and activities may change at any time with or without notice.

Additional Eligibility Requirements

This position may require access to classified information or restricted U.S. Government sites, systems, or information, as determined by the Company and/or applicable U.S. Government requirements. If the position is so designated, your employment in the role may be contingent upon your ability to obtain and maintain the required U.S. Government security clearance or other government authorization, and to satisfy any citizenship or other eligibility requirements imposed by applicable law, regulation, executive order, or government contract requirements. You will be notified if and when such requirements apply.

Affirmative Action/EEO Statement

Castelion is an Equal Opportunity Employer. We are committed to providing equal employment opportunities to all applicants and employees without regard to race, color, religion, sex, sexual orientation, gender identity, national origin, age, disability, genetic information, protected veteran status, or any other characteristic protected by applicable federal, state, or local law.

Castelion is committed to providing reasonable accommodations to qualified individuals with disabilities throughout the application and hiring process. If you require a reasonable accommodation to complete an application, participate in the interview process, or otherwise participate in the hiring process, please contact View email address on click.appcast.io. Requests for accommodation will be considered on an individual basis and handled in accordance with applicable law.

Castelion is committed to fostering a workplace where employment decisions are based on qualifications, business needs, and the ability to perform the essential functions of the role, with or without reasonable accommodation.

EAR/ITAR Requirements

This position requires access to export-controlled information, and as such, employment (or hiring of a contractor) is contingent upon the candidate’s ability to access all applicable export-controlled information without additional export licensing being required by the Bureau of Industry and Security and/or the Directorate of Defense Trade Controls.