Battle Moon Wars

This isn't actually a visual novel... it's a turn based strategy fighter game based on the TYPE-MOON novels (visual and not).
The fight mechanics are pretty much a clone of Super Robot Wars, and for gamers that were old hands with SRW, BMW was easy enough to pick up.

In this story, stagnation is occurring in Misaki City (setting of Tsukihime), and some members of this investigatory government group (I think they're government XD) are sent to determine what's causing it and to help the guardian of that land out. Seeing as the Tohno family is the guarding family of Misaki City, it's Akiha the two agents have to seek out.

Anyway, vampires once thought defeated, servants of the Holy Grail War that was thought to have been ended forever - all these resurface in Misaki, and a collection of characters from Tsukihime, Kara no Kyoukai and Fate/Stay Night (and their related spin-offs/sequels, such as Len from Kagetsu Tohya and Kaleido Ruby from Fate/Hollow Ataraxia) all end up working together to defeat the evil that shouldn't even exist.

While the story is intriguing (there's some really LOL moments, such as when Arcueid sees a mahou shoujo movie and decides she's going to be a magical girl too, under the name Phantasmoon), it is worth noting that all the characters are equal here - there is no 'a servant possesses roughly 1/4 of Arcueid's strength' (although she isn't really fighting as Arcueid, but rather a nerfed mahou shoujo), nor 'only Ciel and Kishima Kouma [of Tsukihime] would be able to fight against a servant in a defensive fight'.
Of course, there isn't really any other way of having a strategy component if just one character could practically god-hack all the characters dead in one turn...

Anyway, there's a translation project of this game in progress, alongside the fact that WERK (the company behind BMW) haven't actually finished all the expansion packs to this game yet... Part 4 is apparently going to come out in August this year.