Holy Grounds in general are uncommon but not unique, and are used to connect to other dimensions, worlds, and space. Normally, a user just needs to know the methods of operating a Holy Ground and he or she will be able to control it. Holy Grounds are magical existences which are said to be 'newer' creations than Vampires. As a Vampire (Most Ancient Sorcerer), technically speaking, Himea has the ability to decipher (like Alpha Stigma) the workings of a Holy Ground and can gain control of it, since it's a magical existence.
However, the one at Miyasaka High is a unique Holy Ground of great power. It can connect to ANY and EVERY dimension, world, and space as long as one can give the coordinates. That's why the Military is so careful in guarding it and camouflaging it as a school, since it's a potential source of great power, as one can use it to learn magic, acquire weapons and knowledge and monsters from other worlds. The Holy Ground in Miyasaka High is also unique in that, it can only be operated by its Contractor it has chosen (and no one else), who is currently Gekkou. Of course, this information is totally skipped in what should have been the event after episode 5 (which they just crunch last chapter of volume 2 and chapters 1, 2 and 3 of volume 3 into that episode), where Gekkou and Himea had a fight as Himea tried to take control of the Holy Ground in the student council room. Furthermore, the only one who can contract it must be 18 years or under. As to why that is, it might be revealed in future so I won't touch on it.
Finally, Himea currently has a Holy Ground that she administers in another space. She has set up that Holy Ground with a fixed portal to the park. That Holy Ground can only connect to a limited number of dimensions, and it's affected by the inter-dimensional distance from this world. And also, Himea did not follow Taito along, since she needs to stay within the school compound with the protective barrier of the Military (but in the anime, she did!). Here, emotions of Himea is also downplayed in the anime when she knew that Taito had gone to Edeluca. In the novel, she was much more frantic and anxious because she knows that the Price for a Wish from the Mistress of Edeluca is never cheap, so she's very worried about the health of Taito and what might happen to him. Taito then first used the portal in the park (with instructions from Himea) to reach the Holy Ground in another space. He then accesses the information given by Mistress of Edeluca and managed to use the Holy Ground to connect to a coordinate available in that fountain of knowledge not meant for human consumption. This is how he got to Nyankichi. I think Nyankichi seriously FTW though I also find the anime version annoying rather than funny. Oh yeah, since Himea did not follow Taito, Nyankichi was never swooned by her or anything of that sort. That's just plain stupid in the anime.
Edeluca is the name of a place by the way. Though initially, I also got it wrong because of the ambiguity in the sentences when Gekkou first mentioned it. And yes, the Mistress of Edeluca took most of Himea's remaining magic as the price for Taito's revival, which is a very big thing, for she is a Vampire (Most Ancient of Sorcerers). By now, it should be clear that except for fangs and red eyes, Vampires are not bloodsuckers, but the name for a magic using race, and the most Ancient type at that. Anyway, this was what weakened her greatly. Continue speculating, it's fun. :) And the 'price' should be revealed in the last one or two episode.
