That was a period of 500 years, which started some 700 years AFTER (!) establishment of the Bulgarian Kingdom.
So Bulgaria just REGAINED its independency, in the 1800s, unfortunately in way-smaller size than it had been in the 14th century.
Bulgaria was ruled by Ottoman Empire of Turkey as recent as late 1800's before it became independent.
for Mussala: 42.1833 lat, 23.5833 long
Another popular starting point for Rila is the town of Dupnica - about 2 hours by train south of Sofia . If you want to hike in the Western part of Rila (including "seven rila lakes"), this is where you need to start.
you pay for camping in Rila Mts when you do it near the challet (appr. 1 euro per tent). Night in the shelter/challet costs ca. 5 euro