Senior Backend Engineer (Node.js / NestJS) – Greenfield Product at Caronsale

Senior Backend Engineer (Node.js / NestJS) – Greenfield Product Freelance Contract | 9–12 months Remote from Brazil About the Project Castle Tech GmbH is building a new standalone product from the ground up and is looking for an experienced Senior Backend Engineer (Node.js / NestJS) to join us on a freelance basis for 9–12 months. In this role, you will play a key role in shaping the product's architecture, backend services, and technical direction. You will work closely with frontend engineers, product managers, and stakeholders to turn ideas into reliable, scalable backend systems. This is an opportunity for engineers who enjoy designing systems, solving complex technical challenges, and taking ownership of meaningful projects from concept to production. What You'll Work On • Build and maintain scalable backend services using Node.js and NestJS • Design a clean system architecture for a new production product • Take ownership of features from concept through deployment • Define and implement APIs, services, and backend infrastructure • Collaborate with frontend engineers, product managers, and designers • Establish development standards, CI/CD pipelines, and engineering best practices • Mentor other engineers through code reviews and technical feedback • Continuously improve workflows using modern tooling and AI-assisted engineering AI-Augmented Engineering AI is part of how we work. We expect engineers to actively use modern AI tools to improve development speed and quality, while maintaining strong engineering judgment. You should be comfortable: • Using AI-assisted coding tools in your daily workflow • Writing effective prompts and critically evaluating AI output • Identifying AI limitations and failure modes • Exploring modern AI-driven development workflows This role is for engineers who see AI as a tool to enhance engineering, not replace good engineering practices. Your Skills Must Have • 5+ years of backend engineering experience • Strong experience with TypeScript, Node.js, and NestJS • Experience designing scalable backend architectures • Strong understanding of software design principles and maintainable systems • Experience with relational databases (PostgreSQL or similar) • Experience working with Git, CI/CD pipelines, and Docker • Experience mentoring or supporting other engineers • Experience working in agile teams (Scrum or Kanban) • Strong communication skills in English (written and verbal) • Availability for 9–12 months, starting as soon as possible • Comfortable working remotely with European time zone overlap (able to attend meetings during CET/CEST business hours) Nice to Have • Experience with microservices or event-driven architectures • Experience building greenfield or standalone products • Familiarity with AI-related components such as RAG or vector databases • Experience integrating AI services or data-driven systems • Conversational German • Previous experience working with European clients or distributed teams What You'll Own As a Senior Engineer, you will play a key role in shaping both the technology and engineering culture. You will: • Design backend systems with clear service boundaries and scalability in mind • Identify technical risks early and propose effective solutions • Contribute to architecture discussions and system design • Improve the team's engineering standards and productivity • Mentor engineers and contribute to technical decision-making • Deliver high-impact backend features aligned with product and business goals Contract Details • Duration: 9–12 months (with potential for extension) • Location: Remote from Brazil • Start date: ASAP • Engagement type: Freelance / Contract (B2B or international contractor agreement) • Work arrangement: Fully remote with expected overlap during European business hours (CET/CEST) • Time zone: Flexible, with core collaboration hours aligned to Central European Time Why Work With Castle Tech? • Build something new – work on a greenfield product from the start • Real technical ownership – influence architecture and system design • Modern engineering environment – use the latest tools and workflows, including AI-assisted development • Collaborative team – work with experienced engineers, product managers, and designers • Remote-first mindset – async-friendly culture with clear communication practices • International exposure – work with a European tech company from Brazil