Apulum was a legionary fortress in the Roman province of Dacia from the 2nd to 4th centuries AD, located in today's Alba Iulia, Romania.

[6] It is the largest castrum in Romania, occupying 37.5 hectares (93 acres) (750 x 500 m).

It was the base of the legion Legio XIII Gemina transferred there by Trajan to the newly conquered province of Dacia at the end of the war in 106.

The city of Apulum grew up around the fortress, eventually becoming the capital of Roman Dacia.

The legion was relocated in 271 to Dacia Aureliana when the northern Dacia province was evacuated This Dacia-related article is a stub.