I think Lego was lucky to tap into the Star Wars collector's market... Cheap robots has been another market the Lego Group have been able to tap into... But here's my idea... to "save" casle -- and I say this tongue in cheekly -- there are thousands of people spending hundres of millions of dollars on World of Warcraft minitures armies... some have found that it's cheaper to build armies in Lego. I say release castle minifig packs again, marketed with these people in line... let the product line have Orcs, Goblins, bring back the Harry Potter Ogre, Elves, Dwarfs... not something that fans have pieced together from different sets over time, but themed this way... Give each minifig a 4x4 green plate to stand on (or a 4x6 plate if the minifig is mounted on a horse), different heads, torsos, legs, at least 1 to 2 different weapon for each minifig, a few helmets, shields, armor... Have a baddies pack, base on the same idea... They'll sell.
Update: TLG has done this to some extent with Battle Pack Knights and Battle Pack Skeletons. All they need now are battle pack dwarves, orcs, and trolls. :)