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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. Castles detection platform — rule engines, counters, threat intelligence lists, ML models — is designed to stay simple enough to reason about, safe enough to iterate on daily, and fast enough to run in the hot path of billions of API requests. You define how it runs. Youll co-own this platform, bridging Platform and Intelligence — shaping how the building blocks are designed, deployed, and monitored. What you care about The research team wont hand you a spec. Theyll describe a detection problem, and youll figure out how to solve it. How does it behave under load? When researchers can ship a new detection rule during Tuesday peak hours without hesitation, thats your work paying off. 8+ years of professional software development in backend-heavy or platform roles — SaaS, API-first, or infrastructure environment. ~ Youve built performance-sensitive production systems — streaming pipelines, in-memory stores, or similar infrastructure where latency and throughput matter. ~ Proficiency in Ruby and familiarity with our core stack: AWS, PostgreSQL, Kafka. ~ Nice to have: ML model serving, security/fraud/bot detection background. This is a fully remote role, with teammates across the US and Europe. We have offices in New York and Cracow. We pay competitive salaries, offer equity, flexible hours, unlimited PTO, and parental leave.