The Main Guard building was built in the central part of the Holy Trinity Square. It was built in 1728 – 1729 as a single storey building with multiple wings. The arcades on the ground floor open it towards the square, and a «thick» bell tower with a terrace rises in the centre of the building facade. Its massive nature and a watchtower with a Moorish-Venetian style dome are evidence that it was used as a military building. The building was restored in 2007 and repurposed as the Archaeological Museum, which was attached to the Museum of Slavonia in 2012.