All the way down in the South of Poland there is a trail with magnificent castle ruins dating from around 14th century, which stretches from Częstochowa to Kraków. They were built by Kaziemierz III Wielki during whose reign Poland become very powerfull. The trail is called Trail of the Eagles' Nests because the castles were placed on top of rocks to make the protection of the land beneath easier.
It is most amazing to walk around the ruins (some castles has been restored, into some you can go in and in some cases you can just walk around), which are so old and which played a very important role in Poland's history.
Photos are better than writing about it ;)