Nutaku has a checkered history regarding censorship. Their removal of many characters from their most popular title, Millennium Wars Aigis, severely disrupted game balance and caused considerable backlash from their fanbase. These characters were removed because they were considered too young-looking for Western audiences. In response to the backlash, Nutaku experimented with other types of censorship that would be better tolerated. Nutaku retained all characters in Osawari Island, instead completely redrawing character faces and bodies to make them look more "mature".
Comparison shots showing off Nutaku's "artistic side"
Nutaku was taken by surprise when fans were still unhappy. Perhaps learning from these incidents, Nutaku made alterations in subsequent titles more subtle. Lolis were given ample breasts, since breast size clearly indicates sexual maturity and readiness for sex. Scenes involving tentacle rape were stealthily substituted with other creative options. You see, tentacles represent bestiality, the exploitation of a creature that is incapable of giving meaningful consent to sex, and therefore tentacle rape would represent statutory rape of the tentacles. Yet still some fans weren't satisfied, and privately Nutaku discussed quietly rolling back these much-maligned alterations.
Yet some things never change. Indeed, Girls Kingdom hasn't escaped the censors' brush. But Nutaku is missing the point, as the alternations themselves were never the main issue. No, it was the dishonesty. You see, Nutaku has not *once*, to my knowledge, informed their audience that a game would be censored, or what alterations were made in advance. They prefer to keep quiet about it and hope their audience doesn't notice, because censorship is frowned upon and some customers would leave over it. But the Internet doesn't like a coverup, and if they won't reveal it, then it will be revealed for them.
I gathered a pair of comparison shots of the DMM and Nutaku versions as a sample of the censorship in Girls Kingdom. These are surely just the tip of the iceberg, and the cuts could very well run much deeper.
Girls Kingdom DMM vs. Nutaku Image Comparisons
In addition, I've compiled a comprehensive list documenting eroge in English with censored official English releases. To my displeasure, Nutaku releases feature prominently in this list.
Eroge with a censored official English release, a comprehensive list
I hope others find this resource useful, as I devoted considerable effort to create it. Other Nutaku games not currently on the list are also likely censored, and I'd welcome community input to document them as well.
Edit 7/29/16: My Imgur gallery comparing 3-4 CG from Nicole and Rosalia's H-scenes in the DMM and Nutaku versions, for the purpose of exposing Nutaku's censorship, has been taken down by a DMCA from Nutaku!
Edit 8/3/16: I've posted a full write-up.
Nutaku caught abusing DMCA takedowns to censor evidence of censorship