Castle Fund
Investment Thesis
Castle Fund is a seed-stage deep tech fund whose organizing idea is that intellectual property is the one thing that can be underwritten when a company has little or no revenue. The firm says IP signals focus, sets margin potential, and shows whether a company can hold its position once incumbents notice. It looks for founders who are securing patents, trade secrets and commercial partnerships, and it treats an IP roadmap that starts on day one (what is a full patent, what is provisional, what stays a trade secret) as a core diligence input. The firm also argues that IP drives enterprise partnerships and that it is a key factor in deep tech M&A, so strong IP can shorten time to exit and capture the outlier outcomes that return a fund.
Sector Focus
Two mature verticals anchor the fund: biotech (therapeutics, devices, diagnostics) and aerospace (space, transport, defense). Around those it invests in the enabling technologies that serve them: energy, robotics and chips. The managing partner's writing adds specific current interests, including organoids and proteoforms in biotech, next-generation batteries and heat-to-electricity conversion in energy, and in-space assembly, manufacturing and next-gen communications in space. He has also argued that a coming pharma patent cliff makes 2026 a strong year to fund early-stage biotech.
Stage Focus and Check Size
The firm describes itself as a seed fund. One third-party description puts Fund I at roughly $20M, and the firm says Fund I has had first closings. Check sizes are not published, so none are recorded here. The first two announced investments were seed rounds, and Arinna's round was led by a prior investor in Boom Supersonic and K2 Space, which suggests Castle takes a participating role in at least some deals rather than always leading. Lead tendency is therefore unknown.
Recent Activity
On 21 February 2026 Daniel Gould announced the first two Castle Fund investments: Sophia Space, which builds modular tiles with proprietary thermal management and distributed compute for orbital data centers, and Arinna, which uses transition metal dichalcogenide materials to make solar panels with about ten times the power density of silicon. The portfolio page also lists Cirrus Therapeutics (gene therapy for dry AMD, shown under warehoused investments), Axiom Space and Rejuvenation Technologies. Since then the firm has published a steady run of essays on IP commercialization, AI and patent strategy, and biotech patent cliffs. Fund status is best read as actively deploying from a first fund.
Portfolio Highlights
Castle's own portfolio is small and new. Sophia Space is led by a JPL veteran who is also a GP of Mandala Space Ventures. Arinna has PhD founders from Stanford and was backed at pre-seed by Breakthrough Energy. Axiom Space is described as the first commercial space station startup, with four missions flown to the ISS and a Series D at a $2B valuation. Rejuvenation Technologies is Khosla-backed telomere therapeutics at Series A. The site's track record section (Redfin, BillionToOne, Jeeves, Chipper, Yassir, Volumetric, Hypoint, Kernal Bio, Array Labs, CytoReason, Quaise Energy, Endiatx, Earth AI, Unspun) is explicitly the managing partner's personal angel portfolio and his Asymmetry Ventures AngelList syndicate, not Castle Fund I, and is not recorded as Castle portfolio.
Team and Decision Process
Daniel Gould is Managing Partner: a former founder and Venture Partner at Asymmetry Ventures, manager of a 5,000-LP AngelList syndicate, and an IP lawyer and business development executive (Greenberg Traurig, then licensing work at Intel, Sony and Amazon; Columbia Law and Stanford CS). Joe Turner is a Venture Partner with Blackstone and biotech CFO experience. Chris Harvey is the fund's attorney, Jake Russell is an LPAC IP advisor, and GP advisors include patent attorney Jaimes Sher, Asimov Press founder Niko McCarty and Band of Angels founder Ian Sobieski. The structure looks like a lead GP with a bench of specialist advisors; Gould appears to drive decisions.
Founder Preferences
The firm describes three lenses. The generalist lens wants relentless visionary founders, scalable unit economics, a big market and speed to market. The IP lens wants an IP-centric roadmap, velocity of innovation rather than raw patent count, partnership moats and interdisciplinary teams. The deep tech lens wants order-of-magnitude KPI improvements, third-party technical validation and clear technology readiness levels.
Geographic Focus
Not stated. The firm is based in Pacifica, California, and its early deals are US-based, but no geographic restriction is published.
Gaps
Check size, ownership targets, lead tendency, fund size and decision timeline are not published and are left unset.