According to the wikipedia.fr article about the château de Montrésor, there were ancient fortifications on the high ground at Montésor before the year 900. The treasurer of the cathedral at Tours held the place as his fiefdom back then, and that's how Montrésor ("my treasure") got its name. Soon, though, it fell under the control of the counts of Anjou, notably Foulques Nerra, and then of the Plantagenets Henri II and Richard Cœur de Lion, future kings of England.
By the year 1200, Montrésor had become part of the kingdom of France once and for all. Toward the end of the 1300s, a noble family from Touraine bought the Montrésor fortress and proceeded to have a chapel and a residential château of some sort built there, turning it into a true château-fort. One of the buildings dating, apparently, back to that period is the one in the photo above, called les communs ("the commons"). It stands across the interior courtyard from the logis Renaissance pictured in yesterday's post.
According to dictionaries, les communs is a term that describes an outbuilding (or une dépendance in France — the British might call it an outhouse!), or a secondary edifice standing alongside the main château or residence on such a property. The commons were the buildings given over to servants' quarters, the kitchens, the stables, and so on. Another meaning of les communs is "outhouse" in the American sense of the term...