The castle ruins thing in Underworld Evolution were done in Zbrush. Luma were interviewed by Pixologic http://pixologic.com/zbrush/interviews/luma_interview.html I'd imagine it could be something similar for games, just using normal maps instead.
Register for free at www.3Dbuzz.com and you can get some free videos here http://www.3dbuzz.com/vbforum/sv_dl_tr.php But it's as Snowfly said, once you understand the very basics you can do loads with just that. In the 'classroom' area of the pixologic site you can see lots of little tricks and things you can do in zbrush.…