Staff Software Engineer - API/Backend

About Castle

Castle is building real-time infrastructure to protect SaaS platforms from fraud, abuse, and malicious automation. Our product stops bots, detects account abuse, and helps teams fight sophisticated attacks without slowing down real users.

We’re trusted by companies like Canva, Atlassian, and Rockstar Games, and backed by Y Combinator, Index Ventures, and top-tier angels from Stripe, Datadog, and New Relic.

The API is the Product

When our API is slow, a gamer can't log in, a team can't access their tools, a marketplace can't process a transaction. Our customers embed Castle in their critical path — performance isn't a nice-to-have, it's the reason they trust us.

Castle’s API isn’t just an integration point — it is the product. Every real-time fraud decision, every detection signal, and every customer workflow flows through it. We handle billions of requests a month under strict latency budgets, with the expectation that it just works.

You'll work on the backbone: the path from incoming API call to real-time fraud decision — token decoding, signal processing, rule evaluation, response assembly. You'll own parts of this pipeline alongside a small Platform team that already has deep coverage on infra, performance, and deployment tooling. Your focus is the layer where platform meets the outside world: the APIs customers integrate with, the backend of our client-side SDK pipeline, and the enterprise capabilities that large accounts depend on.

What you care about

You think about what happens at deploy time before you think about the happy path. You care about backwards compatibility, safe rollouts, and graceful degradation. In a system processing billions of requests, you know that a careless schema change or a missing fallback can cascade into a customer-facing incident.

But you're not someone who only builds what's specified. You have opinions about API design: consistent naming, clear error messages, predictable behavior. You'd rather build one well-designed primitive than three features that each solve half the problem. When something about the interface feels wrong, you fix the abstraction — you don't patch around it.

When a new enterprise customer integrates Castle in an hour instead of a week because the API just makes sense, you know you've built something right.

You use AI tools effectively in your workflow — not as a novelty, but as a natural part of how you write, review, and debug code.

Experience

• 8+ years of professional software development in backend-heavy or platform roles — SaaS, API-first, or infrastructure environments.

• Experience with real-time, large-scale APIs and asynchronous processing — you've built systems where latency budgets are tight and background jobs are a first-class concern.

• Proficiency in Ruby and familiarity with our core stack: AWS, PostgreSQL, Kafka.

• You've shipped public-facing APIs and care about the developer experience around them: documentation, consistency, error handling, versioning.

• Strong written and verbal English communication to collaborate with a distributed team.

• Nice to have: experience with SDKs, OpenAPI, event-driven architectures, or security/fraud detection.

This is a fully remote role, with teammates across the US and Europe. We have offices in New York and Cracow. We expect candidates to be located between UTC-5 and UTC+1 for timezone overlap.

We pay competitive salaries, offer equity, flexible hours, unlimited PTO, and parental leave.