It’s a good day to be the king. Zynga tells us that CastleVille has amassed over 5 million daily players in just six days. (However, AppData tells a different story, though we’re told it will reflect this number in a few days.) That’s leaps and bounds beyond how quickly the developer’s top Facebook game, CityVille, grew. The city-builder took on just 3.2 million daily players in the same time frame when it launched in December 2010. Zynga provided more fun facts.

The San Francisco-based social games giant says that 68 percent of CastleVille fans are logging in twice daily, and that 135,176,035 quests have been completed. Players have expanded their kingdoms in the Gloom 4,594,750 times, all the while bashing 23,845,983 beasties with their bare hands. If anything, that’s one helluva lot of clicks.

CastleVille launched just last week to fanfare, but likely exploded over the weekend thanks to cross-promotions in FarmVille and CityVille–two of the developer’s most popular games. In CityVille, players can earn 20 free Zoning Permits for reaching Level 5 in its medieval counterpart, while FarmVillers will score five Instant Grows for doing the same.

These rewards are far more substantial than usual cross-promotions, and we bet there will be more where that came from. Adventure World failed to grow on its own for its first few weeks of existence, but Zynga stepped in with liberal use of cross-promotions to skyrocket its player base to over 16 million monthly. While the big Z might be more reliant on its existing players than it would like, this growth is nevertheless … insane? Yeah, we think that’s the word.