The first place that Tom meets the Devil is at the ruins of an old Indian fort.
By Indian, the text is using the then-common term for Native Americans. The Devil (called "Old Scratch" or the "Black Woodsman") is drawn to that place because it was dedicated to him by the Indians, and they made sacrifices to him there.
The "fort" if it can really be called that, is said to have fallen into disrepair and is now little more than a series of gradually sinking "embankments" (mounds of earth). Thus there isn't really much that's fort-like about the location, other than its history and the remains of the battles fought there, such as the tomahawk-hewn skull that Tom kicks.
It seems clear that the Devil's power extends beyond this location, but he may simply prefer residing there, or the reasons may be more literary; the location serves as a more powerful grounding of the Devil's character, and by always referring to him and placing him in that location it lends him both a sense of tangibility ("the Devil lives in the old Indian fort") as well as the supernatural element of suggesting that he is bound there, and cannot leave as a normal person might.