Short answers, pulled from the story.
The Battle of Shipka Pass was a series of four battles fought between July 1877 and January 1878 during the Russo-Turkish War of 1877-1878. Russian forces and Bulgarian volunteers known as opalchentsi defended the Shipka Pass in the Balkan Mountains against Ottoman attempts to retake it. The final battle ended with the surrender of the remaining Ottoman forces under Veissel Pasha on the 9th of January 1878.
The Bulgarian volunteers at Shipka Pass were called opalchentsi. They fought alongside Russian forces to defend the pass. Their stand was later commemorated in the ode The Volunteers at Shipka by the Bulgarian poet and writer Ivan Vazov.
Approximately 30,000 Ottoman troops under Süleyman Hüsnü Pasha attacked Shipka Pass in August 1877. The defenders numbered around 5,000 Bulgarian volunteers and 2,500 Russian troops at the height of the engagement. Ottoman losses in the August battle approached 10,000 killed or wounded, while Russian losses came close to 4,000.
Shipka Pass was the main road through the Balkan Mountains linking northern and southern Bulgaria, running from Zistovi through Tirnovo and Eski Zagra to Adrianople. Ottoman control of the pass would have allowed them to threaten Russian and Romanian supply lines, potentially relieve the besieged fortress at Pleven, and force the war into a stalemate favorable to the Ottomans in peace negotiations.
Süleyman Pasha was court-martialed for his failures at Shipka Pass, despite having arrived after the pass was already lost. He was sentenced to death for wasting men and materiel and failing to protect his remaining troops. Sultan Abdulhamid II commuted the sentence, and Süleyman was sent into exile in Baghdad.
The fortress of Pleven surrendered to Russian forces on the 10th of December 1877, freeing as many as 65,000 soldiers under General Gourko. This allowed Gourko to outflank Shipka Pass from the west by taking Sofia and cutting off Ottoman access from behind. The encirclement made possible the final fourth battle, which ended in the complete Ottoman surrender on the 9th of January 1878.