There's a whole section on Blackmoor town and castle. It's said to have a population of about 1,000 peasants, plus 100 soldiers, 100 elves, and a bunch of others (wizards, trolls, a dragons, etc.). Blackmoor was described by Gary Gygax in one of the earlier D&D books (possibly D&D Vol. 3: The Underworld & Wilderness Adventure) as a "one-horse town", and I suppose that by the standards of other fantasy cities it is rather small. In terms of resources it's rich in iron ore and coal, and the main form of livestock is a large type of bison.
The current ruler is Baron Fant, who was placed in charge of the castle after the first invasion by the Egg of Coot. His biggest ally is Sir Jenkins, who rules the northernmost area of the Great Kingdom. He was a former bandit, but achieved great honours after the first Coot invasion, and married Fant's cousin to strengthen the alliance (Fant also married one of Jenkins' relations.)
Blackmoor Castle itself is said to have been built during the third year of the reign of Robert I, "King of all Geneva", primarily as a defense against the barbarians from the north. (The mention of Geneva I take as a reference to the wargaming groups of Lake Geneva in the real world; in universe I suppose it means the Great Kingdom). No exact time frame is given for when the castle was built, but the moat surrounding it was created some 400 years ago by a Wizard named Pissaic. The castle was taken during both invasions by the Egg of Coot, but in both instances it was retaken.
One of those instances happened about 40 years ago, during the Second Coot Invasion. The Keep's ruler at the time, Baron Ra-all the Wise, was promoted to King of Vestfold, and Blackmoor was placed under the rule of someone known as "the Weasel". Blackmoor was besieged by barbarians, and the Weasel decided to try to negotiate a truce. The barbarians sent ten delegates, but through magic unleashed by a wizard, the delegates transformed into hideous creatures and slaughtered the inhabitants of the castle.
Near the Blackmoor walls there are seemingly bottomless pits that connect to underground caves that riddle the area, leading to the netherworld and the dungeons beneath Castle Blackmoor.
About five miles northwest of Blackmoor is a ruined temple that once belonged to the Dark Lords of the Egg of Coot. It was razed about 500 years ago. In the middle of the temple is a great orange jewel on a black pedestal; adventurers have taken it several times, but each time those thieves have met a violent end, and the gem has returned to its resting place. This is expanded on later: the place is also known as the Temple of the Id Monster, the gem is guarded by undead Super-Heroes, and anyone who steals the gem is pursued by the Id Monster, which only they can see. The creature will devour them and take back the gem, and the thief will wake up naked on the town garbage heap having experienced being eaten alive.
Northeast is the abode of the Wizard of the Wood (who was either called Pete, or played by a guy named Pete). His abode is guarded by illusions of hideous monsters, a trio of Ents at the entrance, and a Fire Elemental in the fireplace. "Pete" apparently died after playing for two years, on a trip to the City of the Gods, but his home is still there.
Not far from there lies the Super Berry Wood, a timeless place where all who enter lose track of time, and don't want to leave (anyone inside must make a save vs. charm person every turn). The great "Berrium Maximus" grows there, also known as Super Berries, which are the size of pumpkins and have magical properties depending on the season, the phase of the moon, and the maturity of the berry. Arneson doesn't elaborate further, mostly because his own players haven't figured a lot of this stuff out yet.
Wolf's Head Pass lies to the northeast, leading to forests dominated by Ents and Wood Elves of uncertain allegiance, and then to the southern lands of the Egg of Coot.
The Comeback Inn in Blackmoor has cheap food and lodgings, and is apparently a good place to find rumours, but it has a magical charm that prevents those inside from leaving; only by having someone outside the inn pull the person wanting to leave through the door can place be escaped.
Details are then given about Blackmoor Castle itself. It has a basement and five floors. Many of those rooms are said to be haunted. There is the ghost of Baron Alvarez, who was killed by barbarians, and whose appearance foretells doom. The Lady of Lust is fated to take any man who wanders into her grasp (apparently she was cursed for being an "uncooperative wife" which is uhhhhhhh uncomfortable, let's say). There are ghosts of plenty of other former rulers, most of whom are scary but harmless. A Lord Alfred was caught by his wife with a serving wench, and the gruesome beheading she gave him is replayed in one room. A former ruler known as Balfred the Bald kept a "jungle beast" as a guardian; after he was killed by it the room was sealed up, but when the room was reopened, and the beast was gone. Rumour persists that anyone who has betrayed Blackmoor that enters the room will suffer the same fate as Balfred. These hauntings give the place quite a bit of flavour, but as far as I can tell the maps don't show where they actually take place.