Bastides are fortified new towns built in medieval Languedoc, Gascony, Aquitaine, England and Wales during the 13th and 14th centuries, although some authorities count Mont-de-Marsan and Montauban, which was founded in 1144, as the first bastides.

Some of the first bastides were built under Raymond VII of Toulouse to replace villages destroyed in the Albigensian Crusade. He encouraged the construction of others to colonize the wilderness, especially of southwest France. Almost 700 bastides were built between 1222 (Cordes-sur-Ciel, Tarn) and 1372 (La Bastide d'Anjou, Tarn).

Bastides were developed in number under the terms of the Treaty of Paris (1229), which permitted Raymond VII of Toulouse to build new towns in his shattered domains but not to fortify them. When the Capetian Alphonse of Poitiers inherited, under a marriage stipulated by the treaty, this "bastide founder of unparalleled energy" consolidated his regional control in part through the founding of bastides. Landowners supported development of bastides to generate revenues from taxes on trade rather than tithes (taxes on production). Farmers who elected to move their families to bastides were no longer vassals of the local lord and became free men and the development of bastides contributed to the waning of feudalism.

The new inhabitants were encouraged to cultivate the land around the bastide, which, in turn, attracted trade in the form of merchants and markets. The lord taxed dwellings in the bastides and all trade in the market. The legal footing on which the bastides were set was that of paréage with the local ruling power, based on a formal written contractual agreement between the landholder and a count of Toulouse, a king of France, or a king of England. The landholder might be a cartel of local lords or the abbot of a local monastery.

Responsibilities and benefits were carefully framed in a charter, which delineated the franchises ('liberties') and coutumes ('customs') of the bastide. Feudal rights were invested in the sovereign, with the local lord retaining some duties as enforcer of local justice and intermediary between the new inhabitants— required to build houses within a specified time, often a year, and the representatives of the sovereign. Residents were granted a houselot, a kitchen garden lot (casale), and a cultivable lot (arpent) on the periphery of the bastide's lands. The bastide hall and the church were often first constructed of wood. After the bastide was established, they were replaced by structures of stone.

Scholarly debate has taken place over the definition of a bastide. They are now generally described as any town planned and built as a unit, by one founder. Most bastides were developed with a grid layout of intersecting streets, with wide thoroughfares that divide the town plan into insulae, or blocks, through which a narrow lane often runs. They included a central market square surrounded by arcades (couverts) through which the axes of thoroughfares passed, with a covered weighing and measuring area. The market square often provided the module into which the bastide is subdivided. The Roman model, the castrum with its grid plan and central forum, was inescapable in a region since Roman planning precedents survived in medieval cities such as Béziers, Narbonne, Toulouse, Orange and Arles. The region of the bastides had been one of the last outposts of Late Antiquity in the West.

The main feature of all bastides is a central, open place, or square. It was used for markets, but also used for political and social gatherings. A typical square, (which was probably a model for other bastides), can be found in Montauban. Generally, there is just one square. Saint-Lys and Albias are different because they have two squares, one for the market and one square for the church. The square is also used to divide the city into quarters. Generally, it lies outside the main street (the axis) which carried the traffic. There are three possible layouts: Completely closed: The square does not touch any street. These are very rare; there is one example at Tournay with a size of 70 metres (230 ft) by 72 m (236 ft)). Single-axis: The single-axis design of the bastide makes all roads run in one direction and are parallel. Here and there, there are alleys cut between the roads. The square is placed between two roads. These squares are usually 50 m (164 ft) to 55 m (180 ft) on each side. Grid-layout; usually based on the square in Montauban. Generally the flattest place in the bastide was used for the square.

The church was almost never on the central square but usually at an angle, facing the square diagonally. One of the rare exceptions is Villefranche-de-Rouergue but this one was built two centuries after the square.