Quảng Bình Quan stands beside National Route 1 with traffic streaming past both sides of it, a masonry arch attached to nothing. It was once the controlling gate of a serious fortress. The Nguyễn lords first put a garrison post here under Nguyễn Phúc Nguyên; in 1630 their strategist Đào Duy Từ proposed the wall system that Vietnamese still call Lũy Thầy, the Master's Rampart, and the post became one of its anchors. Gia Long enlarged the works in 1811. In 1824 Minh Mạng had the whole thing rebuilt in brick and stone on the Vauban model then fashionable across Asia — a perimeter of 469 trượng, about 1,860 metres, walls some four metres high, a moat roughly twenty-eight metres wide, and three gates. Of all that, the gate and one stretch of wall survive. The 2000s restoration was not universally admired.

This stretch of coast spent centuries as somebody's frontier. It was contested ground between Champa and Đại Việt until 1069, when Lý Thường Kiệt's campaign settled the question, and it stopped being the far southern edge only after the Trần princess Huyền Trân was married to the Champa king Jaya Sinhavarman III and the border moved on past Huế. Then, from 1558, Vietnam split against itself. The Trịnh held the north, the Nguyễn the south, and the Gianh River became the line between Đàng Ngoài and Đàng Trong for more than two centuries. Đồng Hới sat just behind that line, and its walls existed for exactly one purpose: to stop an army coming down the coast. Geography made the job plausible. Vietnam is at its narrowest here, roughly forty kilometres from surf to mountains.

The French flew from Đồng Hới's airfield in the 1950s, striking at the Việt Minh and at the Pathet Lao across the Lao border. A decade later the same narrowness that made the town defensible made it a target: everything moving south toward the seventeenth parallel had to pass through a corridor a few dozen kilometres wide. On 11 February 1965, American bombing destroyed most of Đồng Hới. B-52 raids continued for years. The Catholic church at Tam Tòa was left as a shell, and its bell tower still stands near the centre of town, deliberately unrepaired. What is remarkable, walking the place now, is not the ruin that was kept. It is how completely the rest of it went, and how little fuss the city makes about that.

After the fall of Saigon in 1975 Quảng Bình vanished into a merged super-province called Bình Trị Thiên, and Đồng Hới became a provincial town in someone else's province. In 1990 the three provinces were separated again and Đồng Hới got its capital status back. The rebuild that followed is the city you see: a grid of low, bright buildings between the Nhật Lệ river and the sand, home to 133,672 people at the 2019 census across about 156 square kilometres. Trains stop here on the North–South line, 522 kilometres south of Hanoi. The airport opened on 18 May 2008 with a flight in from Nội Bài, and Ho Chi Minh City was connected the following July. The climate is tropical monsoon: hot, and in autumn spectacularly wet.

Đồng Hới's fortunes changed when the world worked out what was under the limestone an hour inland. Phong Nha-Kẻ Bàng became a World Heritage site, and on 10 December 1990 a local man named Hồ Khanh, out in the forest looking for agarwood, came across the entrance to a cave nobody had recorded. When British cavers surveyed it in April 2009, Sơn Đoòng turned out to hold the largest known cave passage on earth — over five kilometres long, around 200 metres high and 150 metres wide. The first tourist group went in during August 2013, on a permit costing three thousand dollars a head. Meanwhile the beaches at the Nhật Lệ mouth fill up every summer. On 12 June 2025 Quảng Bình was dissolved into an enlarged Quảng Trị province, and Đồng Hới, no longer a city in the administrative sense, became the seat of the whole thing.