The English historian, John Capgrave, wrote that St Cyngar established a monastery in Glamorganshire with 12 canons dedicated to the Holy Trinity. It is believed this was the beginning of Holy Trinity Church. In 1874, the remains of what appeared to be a monastery were still visible. The parish consisted of Marcross Manor, owned by Sir Philip de Marcross from 1189 to 1200. De Marcross' only heir was a daughter; when she married in about 1250, the manor passed to William le Butler. The church was described as a rectory worth five marks in 1254, and was still described as such in 1535. By 1563, it was noted as having a parson and curate; by 1835, while it remained classified as a rectory, the listing for patron was the Chapter of Llandaff.
The main structure dates from the 12th century; it is believed that the tower was added to the building in the 14th century. The tower, with its saddleback roof, may have been partially rebuilt in the 17th century, from evidence of a difference in masonry above its ridge. The rounded Norman arch at the church entrance dates from circa 1150 and the large font with rolled mouldings dates from the 12th century. There are no aisles in either the chancel or nave. One of the chancel walls has two trefoil light windows believed to be from the 13th century. One of these windows may have been used for the ringing of a Sanctus Bell. The building is constructed of stone with slate roofing, with the roof for the chancel being lower than the nave. Still in the chancel is the lepers' window, where those with contagious diseases could view church services without coming into contact with others.
By the late 19th century, it became evident that the church was in need of restoration work. The rector of Holy Trinity began a fundraising campaign for the work; it took four years to raise the needed funds. By 1892, enough donations were received to allow Kempton and Fowler of Llandaff to begin the project. During the restoration, the architects discovered many items of note which had been hidden by previous renovations to the church. While removing old plaster from the walls, a doorway leading to the rood loft on the north side of the chancel arch was uncovered. The plaster removal also located a 13th-century tomb in a recess of the north nave wall. During the restoration, a tomb in the chancel floor from circa 1200 and the church's original pillar piscina were also located.
When the restoration was completed, the church had been completely re-roofed, the church walls and tower were renovated, new floors were installed along with new pews, a new pulpit and reading desk, as well as a new altar and altar rail. Holy Trinity was reopened in 1897.
The Jan Hus Memorial stands at one end of Old Town Square. The huge monument depicts victorious Hussite warriors and Protestants who were forced into exile 200 years after Hus, and a young mother who symbolises national rebirth. The monument was so large that the sculptor designed and built his own villa and studio where the work could be carried out. It was unveiled in 1915 to commemorate the 500th anniversary of Jan Hus' martyrdom. The memorial was designed by Ladislav Šaloun and paid for solely by public donations.
Born in 1369, Hus became an influential religious thinker, philosopher, and reformer in Prague. He was a key predecessor to the Protestant movement of the sixteenth century. In his works he criticized religious moral decay of the Catholic Church. Accordingly, the Czech patriot Hus believed that mass should be given in the vernacular, or local language, rather than in Latin. He was inspired by the teachings of John Wycliffe.