There was a settlement on the place of Aiud in the time of celts too. In the Roman times it's name was Brucla, and had a castrum above on the mountain. The via Strata crossing Dacia passed here, towards Potaissa (Torda/Turda), Napoca (Kolozsvár/Cluj), and Porolissum (Mojgrád/Moigrad). The settlement - fortified with walls by then - became royal property after the Hungarian conquest.

After the invasion of the Mongols in the 13th century, it was refounded by the Saxons. Aiud was first mentioned in a document in 1239. The first church was built by the Saxons between 1333-35 dedicated to St. Aegidius - from here the name of the settlement too. On it's place today stands the small lutheran church without tower, built in 1866.

The castle in the town centre - built during the reign of the Árpád-dinasty -, was enlarged and rebuilt in the 14-15th centuries. Eight bastions were errected, guarded by the armed forces of the local guilds. The walls were once encircled by rampart and moat. The castle-church - built originally in the 15th century in gothic style - was barroquised in the 19th century. The tower is 65 m high with loop-holes.

The first siege of the castle was in 1437, when the revolted peasants led by Budai Nagy Antal occupied it, but gave it up shortly after the offence of the nobility.

It was burned up on the 17th of September 1600 by the voivode Mihály one day before he lost the battle at Miriszló. The most trying days of the castle were in the time of general Basta, when in 1603 human flesh was sold on the market with "bread" made of ground bark - according to the documents.

After the Tartar invasion in 1658 Aiud became the new regional centre. In 1662 the Academicum Collegium was moved to Nagyenyed from Gyulafehérvár/Alba Iulia/Weissenburg to the order of Apafi Mihály (1632-1690). We can find - between many others - Bethlen János and Miklós, as curators, Pápai Páriz Ferenc, Apáczai Csere János, Csernátony Pál, Áprily Lajos as professors, Thököly Imre, Salamon Ferenc, Barabás Miklós, Kőrösi Csoma Sándor, Bólyai Farkas as students of the Collegium.

In 1704 during the war for independence led by II. Rákóczi Ferenc, the Austrian army occupied and despoiled the town.

During the 1848-49 revolution and war for independence, crowds led by Axente Sever and Avram Iancu killed the defenceless residents (mostly elders, children and women) and set the town on fire.

The mass grave of the 700 residents is marked by a memorial tablet on the right side of the castle entrance.