There are no museums or collections exhibited on the island of Halki Dodecanese. However, if you are interested in history, you can visit the Medieval Castle in Chorio, the abandoned village Halki, situated in the center of the island.
The Medieval Castle was built in the 14th century by the Knights of Saint John and protected a lot the inhabitants, mostly from the attacks of pirates, who were plundering the Aegean from the ancient times till the middle 19th century. The Castle of Chorio has strong and tall walls and stands on an impressive position, right on a top of a rocky hill and from there you can see the whole island.
Another historical site to see on Halki is the Burnt Cave, on the northeastern side of the island, where you can go with excursions boat. The Burnt Cave stands on the slopes of an abrupt rock and was the site of a terrible act of war in the middle 17th century.
In 1658, the Venetian admiral Morozini set the cave on fire to kill all the inhabitants of Halki that had been hidden there. This way, Morozini took his revenge because the inhabitants of Halki had informed the people of Rhodes about his attack and Morozini hadn't managed to conquer Rhodes.