The Struve Arc is a chain of survey triangulations stretching from Hammerfest in Norway to the Black Sea, through 10 countries and over 2,820 km. These are points of a survey, carried out between 1816 and 1855 by the astronomer Friedrich Georg Wilhelm Struve, which represented the first accurate measuring of a long segment of a meridian.
I've never seen any of these sites.
The origins of Tallinn date back to the 13th century, when a castle was built there by the crusading knights of the Teutonic Order. It developed as a major centre of the Hanseatic League, and its wealth is demonstrated by the opulence of the public buildings (the churches in particular) and the domestic architecture of the merchants' houses.
I have never been to Estonia.