Correction:
"Enemy Territory" is not a good game to go along with Trisquel, since it's a
game that recommends the non-free software (and spyware) "PunkBuster".
"Enemy Territory: Legacy" (http://www.etlegacy.com/) - which is an fork from
(/evolution of) the original code of ET, and which claims to be faster than
the original mod, while still being compatible with the original clients and
servers - is a better option, since it doesn't work with "PunkBuster".
http://dev.etlegacy.com/projects/etlegacy/wiki/FAQ#Is-ET-Legacy-compatible-with-PunkBuster
(A stable version of ET:L is expected to be released soon.)