Alchester is the Old English and modern toponym for a small town in the Roman province of Britannia. Its name in Latin is Ælia Castra. It is about 2 miles (3 km) south of Bicester, in the northwest corner of the civil parish of Wendlebury in the English county of Oxfordshire. There is also an adjoining Roman military camp.
Alchester had a strategic location in Roman Britain, sited at a crossroad on the Silchester–Dorchester–Towcester road and the Cirencester–St Albans road (Akeman Street). The area bounded by defences, about 10 hectares (25 acres), is almost square, with the earliest defences consisting of a gravel rampart and one or more ditches; later, a stone wall was added to the rampart. The dating of these two phases is obscure.
The planning of streets approaches a rectangular grid, uncommon in smaller Romano-British towns. Along the main street, aerial archaeology has revealed a number of narrow, rectangular strip buildings. Near the centre of the town lay a building with a central court, surrounded by a portico on three sides. Outside the western defences, excavation in 1766 of what was then a prominent mound known as the Castle uncovered a sizable Roman bath.
Iron Age settlement is attested close to the later Roman town, and Roman occupation of the site began in the Claudian period, possibly in the form of a fort. After the 5th century the place was deserted.