It took me a while to figure out which lighthouse this is since the card bears absolutely no information - and after some extensive googling, seems like I have nailed it! I didn't dare bother Agi during the festival for such a trivial question :D
This is the Korkyra lighthouse (a very cool and for me, unusual name for something located in Croatia) built in 1871 on the island Vela Sestrica in south part of Pelješac channel.
As you can see, from the card, there are rooms as well which indeed you can rent, for around 460 euros per person per night, and in return you should get some peace and quiet - yeah, tranquility does come at a certain price unfortunately...
the stamp is from a set of 4 featuring castles in Croatia - this one shows the Mailáth castle in Donji Miholjac, considered one of the most well preserved castles in Croatia.