The oldest part of Halla Church is the Romanesque nave, dating from circa 1200. The tower is slightly later, while the disproportionally large choir is from the middle of the 14th century, in Gothic style and replacing an older on the same spot. Scholars have concluded that there were plans to replace the whole church with a Gothic edifice, but the builders probably ran out of funds after constructing the choir.
The exterior of the church is thus a mix of Romanesque and Gothic. A few sculpted stones decorate the Romanesque nave. They were possibly made by the sculptor Master Sigraf. Two fragmentary runestones are immured in the choir, one containing the phrase '...men from Lübeck killed...'. The choir portal is decorated with sculptured capitals.
Inside, the nave is dominated by a large painted ceiling, dating from 1697. In the middle of the painting is a depiction of the Final Judgement, and in circles in the corners Saint Peter and Saint Paul. The choir walls are furthermore decorated with frescos, quite worn. They depict apostles and a number of saints, including Saint Martin and Saint George and the Dragon. The frescos date from the 15th and 16th centuries, and some of them have been attributed to the Master of the Passion of Christ.
The church contains two medieval items of interest. It has a triumphal cross from the middle of the 15th century, probably made outside Gotland. The church also has a baptismal font made by the master sculptor Hegvald. It is considered one of the most interesting fonts on Gotland. Its reliefs depict the childhood of Christ but also other subjects.
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Kristiansten Fortress was built to protect the city against attack from the east. Construction was finished in 1685. General Johan Caspar von Cicignon, who was chief inspector of kuks fortifications, was responsible for the new town plan of Trondheim after the great fire of 18 April 1681. He also made the plans for the construction of Kristiansten Fortress.
The fortress was built during the period from 1682 to 1684 and strengthened to a complete defence fortification in 1691 by building an advanced post Kristiandsands bastion in the east and in 1695 with the now vanished Møllenberg skanse by the river Nidelven. These fortifications were encircled by a continuous palisade and thereby connected to the fortified city. In 1750 the fortress was modernized with new bastions and casemates to protect against mortar artillery.