This cave was apparently in use, possibly as a secret Catholic place of worship, during the reoccupation of Jilong from 1664–68. The Dutch had left in 1661 to strengthen the garrison in Fort Zeelandia (Tainan) against an attack by Koxinga. The attack eventually succeeded, pushing all the Dutch from the island in 1662 after a nine-month siege. The Dutch then reestablished a small fort on Jilong in 1664, but factional infighting among the Hollanders and the financial drain of supporting an outpost miles from anywhere quickly told, and the authorities in Batavia decided to disband the undertaking and leave Formosa permanently.
[quote=“John E. Wills, Jr”]Unfortunately for the aspiring historical archeologist, the flatlands where the fortress stood have been turned into a modern shipyard; Professor Iwao Seiichi informed me in a conversation in 1973 that in 1941 or earlier he found on those flatlands remains of old stone walls. The outer coast of the island is very rocky. In the high rocks near the southwest corner of the island there is (or was) a small cave with an inscription over its entrance calling it the ‘Barbarian Dwelling Cave’ (fanyu dong), apparently a fairly recent adaptation or miswriting of ‘Barbarian Writing Cave’ (fanzi dong). In this small cave, about five meters deep and two meters high, Dutchmen carved their initials and other signs, and dates from 1664 to 1667, in the soft rock. Some of these were copied and described by Inô Yoshinori at the turn of the last century. Some time after that, the cave was hollowed out into a regular horizontal cylinder, destroying some of the carvings on the walls, and a recess was added at the far end that probably held an image or a statue. These changes may have been made by some Chinese religious group using the cave as a secret shrine. If this is true, it is a curious echo of its probably function in the 1660s. The clearest of the surviving inscriptions in 1973 consisted of the initials ‘K.B.’; the date 1667; the letters ‘I.H.S.’, which stood for ‘In Hoc Signo [Vinces]’, that is, ‘In this sign you will be victorious’; and a cross. This suggests that the cave may have been used by Roman Catholic members of the garrison for devotions hidden from their superiors and the pastor of the little settlement, a very long way from the schuilkerken (hidden churches) of the attics of Amsterdam.[/quote](Around and About Formosa, pp. 278–9)
The cave is right around 25°09’42"N 121°45’58"E - I haven’t been yet but I’m planning to go and take a look at it and other places around there connected with the Dutch in the next couple of weeks if I can find a non-rainy day (in Jilong… yeah right).
[edited for correct coordinates - thanks Chris]