It is known that Charles Dickens devoted a great deal of time to researching the French Revolution before undertaking writing A Tale of Two Cities (Stanford University, "Discovering Dickens"). Hence, while his historic details are peppered with fiction, he does describe the historic events with a "high level of historical accuracy" ("Discovering Dickens"). Therefore, any details having to do with the French Revolution are most likely historically accurate, and one can simply research the details to verify. One accurately portrayed historical event is the storming of the Bastille on July 14, 1789, an event described in Book II, Chapter 21.
Particularly accurate is the repeated sentence Dickens uses to describe the Bastille, a towering prison in Paris: "Deep ditch, single drawbridge, massive stone walls, eight great towers, cannon, muskets, fire and smoke." It is also true that the crowd cut through the chains of the drawbridge and that the governor of the Bastille, Govenor de Launay, ordered his garrison to fire cannons at the crowd. It is also true that de Launay soon surrendered, letting the crowd storm the Bastille to free the prisoners.
Some details in the first sentence of the final paragraph in this chapter are also true: "Seven prisoners released, seven gory heads on pikes...."
Only seven prisoners were found to be held in the Bastille at that time, and all of those were freed. However, while a number of garrisons were killed, the number may only have been three, not seven. Also, Governor de Launay was indeed captured and beheaded, and his head was indeed carried through the streets on a pike; but, Dickens indicates that seven were beheaded along with seven prisoners being freed, which may not be true.