I am usually not too much into churches as I find them cold, but what struck me in St. Aposteln are the frescoes in the three apse ceilings by Hermann Gottfried. These powerful frescoes bring colors and dynamism to the church. St. Aposteln – whose full name in English is the Basilica of the Holy Apostles – is a grand Romanesque church dedicated to the 12 apostles and that features 3 organs (concerts are sometimes organized).
Hermann Gottfried was a (glass) painter whose work can be admired in other churches in Cologne: to see some of his paintings, you can visit St. Hubertus or St. Norbert; to see some of his stained glasses, you can go to the Great St. Martin Church, St. Maria Magdalena and Lazarus in the Melaten Cemetery or to St. Ursula (make sure to check out those articles as well!).
Close by, you will find the pipe store Peter Heinrichs where, the last time I went, they were selling Gandalf’s pipe. They also organize seminars and tastings (e.g. rum and cigar tasting).