Residence on Tongpan Islet Basalt rocks are everywhere on Tongpan Islet， so residents here use this kind of rock to build houses. As a matter of fact， Penghu area is relatively flat (lava platform-near terrain)， and has no high mountains to avert strong northeast monsoon in winter. That''s why residents built walls with Gulao stone to try to divert such strong monsoon and to enable grains here to grow. Situated at the south， Tongpan Islet is vulnerable to northeast monsoon in winter and southwest air current from the south in summer， which in turn contributes to the so-called "Yiziyin" pattern. Yiziyin-shaped buildings are unique to Tongpan Islet， but actually， they were built with similar construction materials (primarily basalt rock or Gulao stone) and toward the direction where wind is flowing as those in other places of Penghu， but one thing special about Yiziyin is that it stands alone just like a seal and have no Gulao stone walls， plus wall on each side generally has two doors and one small window to prevent strong northeast monsoon. Such monsoon is unavoidable even though Tongpan Islet is at the south. Yiziyin Residences on Tongpan Islet are listed as rare heritage by the government， and are under proactive protection in a hope to keep this kind of cultural relics.