From the memoirs of my childhood experiences: “Betrayal from within” - The rise and fall of earthly kingdoms
I grew up in a very historical city - Veliko Turnovo, the first capital of the Second Bulgarian state (12th -14th Century). Earlier, Tzarevetz fortress (pic below) was the actual city where close to 3000 people lived, including the patriarchs and the kings. This castle stood unconquerable for centuries with its three rings of fortification walls and an abyss around the main entrance.
In 1393 the Ottoman Empire besieged the city from all sides and conquered it. The story was a traitor within the kingdom was bribed to leave one of the gates open and lower down the bridge in order for the Ottomans to come inside and conquer the city. For the next 500 years Bulgaria was under the rule of the Ottoman Empire and was finally liberated thru series of events in the Russo-Turkish war, and re-established as a Sovereign State on March 3, 1878.