Credits
Chess pieces
The chess piece images are the "cburnett" set by Colin M.L. Burnett, licensed under Creative Commons Attribution-ShareAlike 3.0 (also available under GFDL, BSD and GPL). They are the same pieces used by Wikipedia and Lichess. Source: Wikimedia Commons.
Stockfish chess engine
The Master computer level, hints and the free game analysis use Stockfish 19 lite (stockfish.js 19.0.1), running entirely in your browser. Stockfish is free software, licensed under the GNU General Public License version 3 (GPLv3).
- Stockfish © T. Romstad, M. Costalba, J. Kiiski, G. Linscott and other contributors.
- stockfish.js (the browser version) © Chess.com, LLC and Nathan Rugg. Lite neural network by Chris Bao (sscg13).
- Complete source code of the exact version we use: github.com/nmrugg/stockfish.js (v19.0.1). Original Stockfish project: github.com/official-stockfish/Stockfish.
- We use the engine files unmodified (
/assets/stockfish/). You may copy, change and share them under the terms of the GPLv3.
If Stockfish can't run in your browser, the site switches to the CastleEarly engine and tells you.
Everything else
The CastleEarly engine (Easy, Medium and Hard levels), interactive boards, lessons and design were made for CastleEarly.