The Karlskirche, or St. Charles Church, is one of Vienna's greatest buildings. Commissioned by the emperor to thank God for an answered prayer, the splendid Baroque edifice was also designed to glorify the power and rights of the Habsburg Empire.

In 1713, the Black Plague swept Vienna, and Emperor Charles VI made a vow: if the plague abated, he would build a church dedicated to his namesake, St. Charles Borromeo. St. Charles was a 16th-century Italian bishop famous for ministering to Milanese plague victims. The emperor's prayer was answered, and construction on the church began in 1715.

The Karlskirche was built in the early 18th century on what was then the bank of the River Wien and is now the southeast corner of the park complex. The Baroque master Johann Bernard Fischer von Erlach did the original work from 1716 to 1722.

After his death in 1723, his son, Joseph Emanuel, took over, completing it in 1737. The lavishly decorated interior stands as a testament to the father-and-son duo. J. M. Rottmayr painted many of the frescoes inside the church from 1725 to 1730.

The ambitious design by the elder Fischer combined architectural elements from ancient Greece (the columned entrance porch), ancient Rome (the Trajanesque columns), contemporary Rome (the Baroque dome), and contemporary Vienna (the Baroque towers at either end).

When it was finished, the church received mixed reviews. It never received the sincerest form of flattery – imitation – and it was regarded as something of an architectural curiosity throughout Europe. Notwithstanding, especially when lit at night, the building is magical.

The enormous freestanding columns were conceived with at least two functions in mind: first, to portray scenes from the life of St. Charles, carved in imitation of Trajan's triumphs to emphasize the imperial nature of the building; second, to symbolize the Pillars of Hercules, suggesting the right of the Habsburgs to their lost Spanish territory. Also outside is a sculpture by Henry Moore in a little pool.

The green copper dome of the Karlskirche rises 236 feet high, a dramatic landmark on the Viennese skyline.

The main interior of the church utilizes only the area under the dome and is surprisingly conventional given the unorthodox exterior. The space and architectural detailing are typical High Baroque; the fine vault frescoes depict St. Charles Borromeo imploring the Holy Trinity to end the plague.