Why it's the capital
Russian forces freed Sofia from Ottoman rule on 4 January 1878. Marin Drinov then proposed it as the capital of the new Bulgarian state, and it was accepted as the capital on 3 April 1879.
Capital of BulgariaSofia lies in the Sofia Valley at the foot of Vitosha mountain, in the western part of Bulgaria. The valley sits at an average height of about 550 m.
Russian forces freed Sofia from Ottoman rule on 4 January 1878. Marin Drinov then proposed it as the capital of the new Bulgarian state, and it was accepted as the capital on 3 April 1879.
Aim for the west of Bulgaria, toward the borders with Serbia and North Macedonia and far from the Black Sea. Skopje is about 170 km to the south-west.
Sources: Wikipedia: Sofia, Wikipedia: Alexander Nevsky Cathedral, Sofia, Wikipedia: Bulgaria