Based in TSCP
Caligula use the same board data structure that TSCP for generation of moves, also for attacks, see and others, he use mailbox 12x10. Search is improved with techniques such as null move, extensions, prunning, LMR, hash tables, etc. Evaluation is also better with mobility, enhaced security of the king, etc.
Caligula uses PolyGlot format book by Fabien Letouzey. Thanks to Michel Van den Bergh for providing free code to use that format. Caligula.bin is ProDeo opening book by Ed Schroder and Jeroen Noomen.
In the endgame, Caligula can access to 5 men bitbases of Scorpio. Thanks to Daniel Shawul.