I found this story via T Vtropes
, having never played Dungeon Keeper and only watched the first three episodes of Sailor Moon before moving on, so I can't comment on Dungeon Keeper Ami's fidelity to either. That said, the author does a very good job of quickly establishing Ami's abilities in a way that makes sense without it feeling like an info dump, meaning even people like me can enjoy the story.
The story itself is not the pinnacle of literary achievement. It is very obviously a fanmade work, complete with technical issues, uninteresting side characters, weak character development, and forgotten plot points. However, it also has the ability to capture and hold your interest such that you will want keep reading until the end, despite the door-stopping length. It's quality comes from placing a very stereotypical character in an equally stereotypical but diametrically opposed position, simultaneously deconstructing both.
It also examines what happens when a guile hero is also incredibly powerful. By virtue of being Sailor Mercury, Ami has powers beyond a normal Keeper's. However, she is limited by her nature, which forces her to find new ways to do things. She may be powerful enough that, were she to do things traditionally, she would likely win, but it is far more entertaining to watch as she finds new ways to use, misuse, and abuse every ability she is given.
What is remarkable is that even as the plot spirals out of control and the setting grows to immense sizes, the story's quality only increases. Many authors have trouble juggling complex and competing agendas, but Dungeon Keeper Ami's writer seems to have the reflexes to do it, resulting a world which, despite being populated by underdeveloped and uninteresting characters, feels very much like it could be real.
The story is very, very plot driven with little time spared for character development, and Ami's newest scheme is more interesting than the characters involved, but you won't notice or care about the problems because you'll be too busy reading the next chapter. Given the story's length, I suggest bringing snacks.