The first fortress in the town was erected in 1492 upon order of Transylvanian voivode Ştefan Báthori. The citadel was built by the strengthening of a fortified church, including a 5000 sqm area in the Southwestern part of the actual territory of the citadel, known as "Small Castle" (Kis-Kastély in Hungarian). It was accomplished somewhere between 1602 and 1652 under judge Borsos Tamás.
From the old fortifications have been preserved the Small Tower and the Square Tower. On 10 August 1601, Basta's mercenaries destroyed the town and the fortification as a punishment for the support the citizens gave to Michael the Brave. The fortification was destroyed again on May 30th 1602, when 400 of mercenaries led by Moise Secuiul and Németh Gergely enter the city. In August 1602 the inhabitants of the city, at the initiative of Borsos Tamás, go to strengthen the destroyed defense, making wooden palisades.
In September 8, 1602, the church is strengthened again, the goals being transformed into shooting holes. In 1605, Borsos Tamás obtain from Prince Ştefan Bocskai an agreement to build a new citadel. The new one was done in time, with towers of different shapes and sizes, adapted to new requirements imposed by increasingly heavy firearms. The first stronghold, at the gate (1613) still preserve medieval defense systems (pit-trap, pitch holes, ditch water). The Tanners' Bastion (1620) is similar to the urban bastions of the fortifications in eastern Germany and Austria. This important work was completed in 1652 and in 1657, 434 local craftsmen strengthening the fortress. The Târgu Mureş Citadel is the last urban fortress supported by the citizens who payed different amounts of money and worked themselves at the buildings.
After the occupation of Transylvania by the Austrians, it became the headquarters of the military garrison based in the town, several buildings being demolished (mid 18th century). In the mean time the Baroque style building was built (on the left hand side of the road in front of the entrance gate) and in the second half of the 18th century the construction works of the ‘barkey’ were started, an addition finished in the 19th century.
The monument has 1.20 m thick walls, (1.40 m the bastions), with holes for hot oil, with elongated slots to allow shooting different angles of pull or slit-shaped hole key reversed for light guns. This defensive system is completed by a pit trap under the gate bastion, water ditches with mobile bridges and a watch road for soldiers, on brick arches and wooden poles. The citadel has seven bastions, bearing the names of the guild which – according to tradition – supported its maintenance: the tanners’, the tailors’, the butchers’, the ironmongers’, the coopers’, furs', locksmiths'.
Today, the citadel became a living monument because of the numerous events organized here. A museum center was opened in the gate fort, presenting the history of the town and of the Citadel.