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A war memorial for the 1592-1593 naval battle against the Japanese. Next door lies a shrine to Admiral Yi who lead the Korean forces to victory, plus a small museum explaining the details of the war and Korean naval strategies. English explanations exist, although when browsing try to keep count of how often you read extremely biased language (answer: a lot).
해금강
Designated "Korean National Scenic Site #2, this is a must-see location. Essentially just a locally famed island consisting of tall cliffs. However, the ferry cruise around the island will thread its way through the extremely thin cliffs leading into the island into a sea cave before backing out and departing through different route. Spectacular. Round-trip 3-hour cruises including both Haegeumgang and a 3½-hour stopover on Oedo from Jangseungpo.
외도
Typically included in cruises to Haegumgang, this is Korea's first privately owned island and host to the Oedo Botanica. It's somewhat of a cross between a botanical garden and one of East Asia's infamous "country-themed theme parks" and designed to make you think you're somewhere in the Mediterranean. Beautifully landscaped and covered with Greco-Roman statues. It's a bit prettier than the brochure describes as the gardens follow a tight valley surrounded by sheer cliffs. Take a detour to the bathroom as the off-the-path views from outside a couple of them are among the best on the island. Conceived by a couple that crossed the 38th parallel after the war quite literally penniless. Worth the trip and has a restaurant and coffee shop at the highest point that mostly only serves tea (Udon ₩4000, coffee ₩4000, tea ₩2500).