For one of the best views in Vietnam, head to Cannon Fort, where there are astounding panoramas of Cat Ba Island's jungle-clad hills, the harbour and the karst-punctuated sea. The entrance gate is a steep 10-minute walk from Cat Ba Town, and it's then another stiff 20-minute walk to the fort, or take a xe om from town (15,000d). The fort was inaccessible in 2019 due to construction work on a nearby hotel, but should reopen soon.
Well-labelled paths guide visitors past underground tunnels and two well-preserved gun emplacements, out to two viewpoints overlooking the island. There's even a cafe (with more great views from its terrace) and a tiny museum.
The tunnels and gun emplacements were first installed by the Japanese in WWII, but were also utilised by the French and Vietnamese during subsequent conflicts.