XML file format (but with 99% of the action of interest in a domain-specific language on the CsInstruments and CsScore elements) that can be played and the reference implementation. Offers complex effects out-of-box apparently.
Allows you to easily define instruments with seemingly arbitrary mathematical functions, and then use them to play notes at given time intervals.
The instrument functions can be parametrized, and each note played can have different parameters.
The instrument definition actually defines a block diagram graph, much like a hardware synthesizer would.
Csound is so not-bloated that it contains an UI system. And it includes an interactive virtual MIDI keyboard that interacts with parameter knobs: www.csounds.com/manual/html/MidiTop.html
If only it had been written in Python... the array manipulation boilerplate would be likely perfect for NumPy, and this would have been exactly what Ciro Santilli wanted!
CSound states that one of its design goals is backward compatibility, and it shows. Some of the stuff is utterly arcane, e.g. you have to remember what GEN10, GEN11, etc. mean instead of having named enums.
It just worked on Ubuntu 20.04 no questions asked:
iainmccurdy.org/csound.html about 100 CC BY-SA examples. Each is a minimal study showing a specific technique, not a full composition, some seem advanced. Dude's a beast.
Documentation-wise, it's a bit lacking. The only dude who can explain it really well, Dr Richard Boulanger, made the "The Csound Book" closed source, so, congrats, this will forever hurt the popularity of Csound.