Thereafter, they would be threatened by the Quadrilateral of fortresses--Rustchuk, Shumla, Varna, and Silistria.

Three of these were connected by railway, which enabled the Turks to send troops quickly from the port of Varna to any position between the mountain stronghold of Shumla and the riverine fortress, Rustchuk.
Even the non-military reader will see by a glance at the map that this Quadrilateral, if strongly held, practically barred the roads leading to the Balkans on their eastern side.

It also endangered the march of an invading army through the middle of Bulgaria to the central passes of that chain.

Moreover, there are in that part only two or three passes that can be attempted by an army with artillery.

The fortress of Widdin, where Osman Pasha was known to have an army of about 40,000 seasoned troops, dominated the west of Bulgaria and the roads leading to the easier passes of the Balkans near Sofia.
These being the difficulties that confronted the invaders in Europe, it is not surprising that the first important battles took place in Asia.
On the Armenian frontier the Russians, under Loris Melikoff, soon gained decided advantages, driving back the Turks with considerable losses on Kars and Erzeroum.