Castle Grimstead was built in the 800s Dalereckoning (about 500 years ago) as an outpost against the marauding drow of the Twisted Tower and within a catapult's throw of that imposing edifice The castle was built by the Glittering Band, an adventuring company given the charter to "domesticate the land and drive back the dark hordes." The outpost lasted for 30 years, a testament to the strong will and strong magics of the Glittering Band. Its original name was Greatstead but gained its present name over the next 30 years, when continual raids and attacks brought it low.
At length the castle could not withstand the combined assaults of the aboveground drow force and infiltration from below, and the Glittering Band fell in combat, their treasures and powerful magics spirited away by the drow to their underground homelands. The castle was partially destroyed by explosions during this final battle, and with the passage of time whole sections have decayed.
North of the castle ruins is Cavenauth, a cavern entry to the Underdark beneath Shadowdale. Original maps (made at the time of the Glittering Band) listed this as a "cave mouth," but later editions corrupted the name and gave it its present form. With the taking of the Twisted Tower in 906 DR, portions of the Cavenauth cave complex were sealed off from the rest of the Underdark, and the area secured against further drow incursions.
It is unknown whether the complex remains sealed or if new entrances have been breached into the drow territories. Cavenauth is often used as a lair by wandering beasts of the forests, and wyverns have been reported to live in the area. As for Castle Grimstead, its walls pitch at perilous angles and most of the original wooden beams and roof timbers have rotted away. It has no inhabitants and no heirs to the Glittering Band have laid claim to it, though the townspeople would welcome someone rich enough to effect repairs (and foolish enough to encamp directly above a suspected Underdark entrance).