Oldtown is the second city of the Seven Kingdoms, and by far the oldest major city on the continent. It is located in the far south-west of the continent, at the mouth of the Honeywine River where it opens into Whispering Sound and the Sunset Sea beyond.
Oldtown is ruled by House Hightower, who hold fealty to House Tyrell of Highgarden. It is notable as the site of the Citadel, the home of the order of maesters. It is also the site of the Starry Sept, the former home of the Faith of the Seven in Westeros before it was moved to the Great Sept of Baelor in King's Landing.
In the books
In the Song of Ice and Fire novels, Oldtown was the largest city in Westeros before the Targaryen conquest. It survived for so long by canny politicking by the ruling Hightowers, who trace their ancestry back to the First Men. When the Andals invaded, the Hightowers (then kings in their own right) welcomed them openly and swore fealty to the King of the Reach. Oldtown's pre-eminence was confirmed when the maesters constructed the Citadel within the city and the Faith of the Seven built the Starry Sept there.
During the invasion of Aegon the Conqueror, the High Septon of the time prayed and fasted for seven days and seven nights on whether the Hightowers should resist Aegon's army. He received a vision showing Oldtown's destruction should they attempt this. At his recommendation, the then-Lord Hightower threw open the city gates and welcomed Aegon into the city with open arms. Aegon dated the first day of his reign to his welcome into Oldtown.
Oldtown is almost as large and populous as King's Landing, but is far cleaner, more beautiful and more efficiently-run. It is famed for is meandering, cobbled streets and many fine inns and septs. It has an immense port which plays host to ships from across the Seven Kingdoms, as well as the Free Cities and even the distant Summer Islands. The port and the entire city is dominated by the High Tower, a massive, seven-sided, stepped lighthouse rising from Battle Island. More than 800 feet high, it is the tallest structure in Westeros, higher even than the Wall. It serves as House Hightower's keep, as well as a lighthouse.