The package is available on https://aur.archlinux.org/packages/castle-engine-git/ . Follow the standard AUR installation process:
Do it manually following AUR docs, like
wget https://aur.archlinux.org/cgit/aur.git/snapshot/castle-engine-git.tar.gz tar xzvf castle-engine-git.tar.gz cd castle-engine-git/ makepkg -sri
Or use a helper like paru. Once you have it working, just use
paru -S castle-engine-git.
Thanks go to Kagamma (Trung Le) for doing this!