Deep in the heart of the middle ages, life was pretty rough in South-West France. In an effort to bring a bit of stability to the area, 'new towns' were planned and built.
These towns were built around a strict grid layout, and also usually fortified, and aimed to bring a bit of stability and security to the inhabitants while also adding to the strength of the respective sides (English and French) in the region.
Some bastides had a more specific military purpose, and were built as a result of 'tensions' during the hundred years war. Hence many are found between the Dordogne and Garonne rivers. Many changed hands between the English and French, some several times, during this period.
The layout of a typical bastide town includes a central square, several large streets running from the square to the edge of the town, and a grid pattern of narrow passages between these main streets. The central square historically had a sheltered hall in the middle for market days, and a series of arched passages around the edges.
The towns offered a degree of safety, tax concessions and exemptions from military service to their inhabitants, and a small plot of land on which inhabitants could build a house.
The churches in bastide towns were often also used for defensive purposes, and designed and built with that in mind.
Of course, some 700 years or so later, these towns preserve their original form to varying degrees. Some have become sprawling large towns, others have largely disappeared, but the area has a good number of towns that have passed the centuries largely intact.
I consider the 'centre' of the bastide area to be the Monflanquin - Monpazier - Villereal area in northern Lot-et-Garonne, since the area has a particularly high concentration of bastide towns within easy reach of each other, including the three mentioned above - Villereal, Monflanquin, Monpazier - plus others including Eymet, Villeneuve-sur-Lot, Tournon d'Agenais, Beaumont, Castillones and Domme. Other areas of south-west France would claim a similar distinction, however...