Series of 4 stamps (rationale, sites are 8, some of them hard to put on a stamp):
1) Banská Štiavnica - an important medieval silver mining site. Stamp design - either Calvary (a small church on top of the hill, the most characteristic sight) or the Trinity column on the Trinity square
2) Levoča and The Spiš Castle - either the castle ruins, largest complex in Central Europe, or a detail from the beautiful gothic Levoca altar by Master Paul
3) Bardejov city centre - an aerial view as on the photo?
Or a detail from a Slovak Karst cave if someone can transfer this to a stamp...
4) Carpathian Wooden Churches - one of the churches, I propose Hervartov - https://en.wikipedia.org/wiki/File:Hervartov_dreveny_kostelik_Slovakia_3834.JPG
or Saint Michael -
More info e.g. here: