The beads of the Caribbean Sea - San Juan National Historic Site in Puerto Rico has a very special place, that is San Juan, the city is coastal, divided into new and old two cities, the new city is full of high-rise buildings, everywhere is modern buildings, But what I remember is the old city, which is only a ruined wall. The military base is said to have been built in 1511 and is similar to a normal castle, but because it is built on the island by the sea, it is an important military base at the time. Walking on the edge of the city wall, the remaining foundation can also see the position of the city wall. There are still a few rusty cannons on the platform on the seaside facing the sea. I stand near the cannon and listen to it. I can hear the rumbling gun sound, and then listen carefully. It was like the sound of waves beating against the rocks. There was also a small museum inside the three flag castle, which we didn't visit because of the rush of time. While hanging around, we saw some of the arches on the ground floor from time to time, which seemed to be a warehouse or a tunnel. Three flags were inserted in the fort, in addition to the Star-Spangled Banner of the United States, the Spanish Army Banner, and the flag of Puerto Rico representing independence. Three different flags were fluttering in the wind, as if telling people about the glory of the year. ★ Open time: Open at 9:00 am~end at 6:00 pm ★ Tickets: USD7/person Tips: The place is not very big, but it is suitable for leisurely strolling. If you are lucky, you can also encounter a free tour bus.