As tomorrow is the final day of filming for Game of Thrones in Iceland, they are already beginning their filming in Croatia. Local Croat reporter Mario Jurič tweeted a collage of some set photos including set building in Iceland as you can see below:

For the set building they will be using an old abandoned hotel in Dubrovnik, Croatia. Also another tweeter posted a higher resolution photo (props to the tweeter):

Comment below if you think you know what they are building!