Overview

Castle is an active, developer-first fraud prevention platform founded in 2015 in Malmö, Sweden by Johan Brissmyr (CEO) and Sebastian Wallin (CTO).The company helps online businesses detect and block bots, account takeovers, fake accounts, and API abuse by analyzing hundreds of behavioral signals through machine learning.

Castle entered Y Combinator's Winter 2016 batch, raised a $2M seed round from First Round Capital in late 2016, and closed a $9.2M Series A led by Index Ventures in February 2019.As of early 2026, Castle lists as active on its YC profile with approximately 42 employees and offices in San Francisco, Malmö, and Kraków.

This report is a company profile, not a failure post-mortem — Castle shows no public signals of shutdown or distress.The core thesis of the company is that security responsibility should shift from end users to the businesses that serve them, with Castle providing the infrastructure to make that shift practical for any engineering team.

Founding Story

Castle's origin traces directly to a prior failure. In 2014, Johan Brissmyr co-founded Userbin, a consumer-facing authentication service for web applications. The product did not survive. Brissmyr later reflected on why: "The consumer authentication space is a difficult market to break into as there are many choices, including open-source options."^[1] Rather than abandoning the authentication problem entirely, Brissmyr extracted a more durable insight from the failure — the problem was not authentication itself, but who bore responsibility for it.

That reframe became Castle's founding thesis. In 2015, Brissmyr and his co-founder Sebastian Wallin, both engineers, started Castle in Malmö, Sweden.^[2] Wallin served as CTO while Brissmyr took the CEO role.^[3] Brissmyr also brought prior experience from SettleBox, a portable digital identity product for peer-to-peer marketplace transactions on platforms like Craigslist and eBay, giving him repeated exposure to identity and trust problems across different contexts.^[4] He holds a Master of Science in Computer Science from Lund University's Faculty of Engineering.^[5]

The founding vision was explicit about the responsibility shift. As Brissmyr wrote in 2019: "We started Castle because we wanted to figure out a way to shift security responsibility from end users to the companies who serve them."^[6] He described the early days in Sweden with characteristic directness: "In those early days as two engineers in Sweden, we recognized that there was an opportunity to turn every online company into a security guardian who could keep their users safe."^[7]

In early 2016, Brissmyr and Wallin relocated from Malmö to San Francisco to participate in Y Combinator's Winter 2016 batch.^[8] Castle presented at YC W16 Demo Day 1 in March 2016, receiving early press coverage from TechCrunch, which described it as "a drag-and-drop account takeover protection solution" for developers.

The product positioning evolved from a broad "account protection" framing at launch into a more specific developer-first fraud prevention platform over the following years, but the core thesis — businesses as security guardians — remained consistent from founding through at least 2023.

Timeline

• 2014 — Johan Brissmyr co-founds Userbin, a consumer authentication service; the company fails due to market crowding and open-source competition, directly informing Castle's founding thesis^[9]

• 2015 — Castle is founded in Malmö, Sweden by Johan Brissmyr and Sebastian Wallin^[2]

• January 26, 2016 — Castle launches on Product Hunt, receiving 369 upvotes and 34 comments; Justin Kan among notable commenters

• March 2016 — Castle participates in Y Combinator Winter 2016 batch; founders relocate from Sweden to San Francisco; Castle presents at YC W16 Demo Day 1^[3]