No, nothing like one. You may be confused because nobody but Order members could see the place, and it was Unplottable as well so it would never show up on a map; however, it was very much a part of the real world. 12 Grimmauld Place was just a fairly ordinary London home, with a couple of enhancements given to it by its Wizard occupants, and (under the Order) protected by the Fidelius Charm.
This last thing was the biggest; it made the place literally impossible to see or talk about with anybody who wasn't let in on the secret specifically by its Secret-Keeper (who was Dumbledore, until he died). So in book 7, when the Death Eaters show up outside it looking for the three kids who keep saying Voldemort's name, they know something's there but just plain can't see it. One can only assume that whatever defensive-spellbreaking properties the Taboo on the Voldemort name had, it wasn't strong enough to penetrate a Fidelius Charm. This squares, too, with the fact that Tom Riddle (I prefer to use his real name rather than the name he gave himself) wasn't able to find James & Lily Potter until Pettigrew was made Secret-Keeper and told him where they were hiding. Of course, the Taboo he presumably had on the Voldemort name back then may or may not have had the same properties as the Taboo as it appears to work during Deathly Hallows, but this is neither here nor there.
For my opinion on the OP, I have to say no- Tom Riddle was probably about 17th in terms of raw spell-power, while Dumbledore could be about 19th (depending on what bonuses you think the Elder Wand grants). But the magic system in the Potterverse is totally different from the D&D magic system, so it's difficult to judge. Many things the D&D rules take for granted are simply disallowed, or made inordinately difficult, in the Potterverse (in particular, resurrection, simple flying before Riddle figured out whatever he did regarding it, and basic Evocations like Fireball).
