Trinity Church with its notable Round Tower was started in 1637 by King Christian IV, the builder king responsible for many of the distinctive Renaissance buildings still standing in Copenhagen (for example, Børsen and Holmens Kirke). The architect was Hans Steenwinckel the Younger. The tower was completed first, in 1642, and the church itself was dedicated in 1656, after the king's death. It was built for the students living in Regensen and belonged to Copenhagen University until 1869. The famous tower, intended as an observatory, is unique in architectural history. A cylinder 15 metres in diameter it contains a broad spiral paved ramp suited to both pedestrians and vehicles. On the outside, pilasters running up the full length of the tower to a round-arched frieze. The window openings rising vertically between the pilasters consist of twin round-arched windows within slightly pointed niches. In other words, outdated Romanesque and Gothic survivals well into the Renaissance. The top windows are a little smaller, having to fit into the arches of the frieze. Inscription with Christian IV's monogram and the year of completion of the tower.

View of Copenhagen from the top of the tower (on the Tower's website).

The portal in the tower was added a little later. It bears the monograms of Frederik III and Christian V. The walls of the tower and church are constructed in a pattern of red and yellow bricks known as Flensborg bricks (thinner than normal bricks). The church stands on a granite plinth and consists of the nave and choir in a single structure with a three-sided choir ending. An upper floor housed the university library (which moved out in 1861). Buttresses separating the bays. Each bay consists of a tall slightly pointed window and a smaller window above that. The upper windows were originally square but were changed in c.1869 to the current shape, identical to those in the tower. The copper-clad belfry dates from 1731.

The interior consists of nave and aisles of equal height, with octagonal piers supporting the beautiful vault. The original interior furnishing was nearly all lost in the great city fire of 1728 and what one now sees mostly dates from the 1730s. The galleries in the aisles are from 1731. The pulpit and altar piece in their highly ornamented Baroqe design were made by Friedrich Ehbisch, also in 1731. The gilding of the vault also dates from that period (although renewed). The large Rococo clock is from 1757.

The small marble organ near the altar is modern, designed in 1987 by Inger og Johannes Exner. The Baroque facade of the great organ at the west end is from 1731.

Trinity Church was originally built for the students living at Regensen, a building opposite the church consisting of four wings around a courtyard. "Collegium regium", called Regensen, is one of the oldest student residences in Copenhagen. It was built in the 1620s, but most of it was burnt down in the great city fire of 1728. Only the two lower floors of the wing along Store Kannikestræde survived (first picture) and the rest was rebuilt in 1731-49.

The wings of Regensen facing Store Kannikestræde and Købmagergade. (Copenhagen Cathedral can be glimpsed at the end of the street in the first picture.) The bust of the famous astronomer Tycho Brahes stand by the Round Tower in the middle picture.