Cologne appears in Frankenstein very briefly during Victor and Clerval's travels down the Rhine. Victor says "Beyond Cologne we descended to the plains of Holland; and we resolved to post the remainder of our way; for the wind was contrary, and the stream of the river was too gentle to aid us" (Vol III Chapter I). Although this passage is very brief and I almost completely looked over it, after doing a bit of research I found some interesting facts about Cologne that could possibly explain why Shelley chose to mention it here.

In the early 1800s, Cologne was highly famous for its cathedral, one of the largest Gothic churches in northern Europe. The church was so detailed that according to Encyclopaedia Britannica the cathedral was started in 1248 but was still unfinished until the ninteenth century. I think with the combination of travelers during this time seeking out the unfinished cathedral and Shelley deeming it worthy of being included maybe the city could be interpreted as a symbol of incompleteness within Victor's role as a creator. 

The photo above is the Roman Catholic Cathedral in Cologne, Germany found in the public domain courtesy of Bing.

“Cologne Cathedral.” Encyclopaedia Britannica, Encyclopaedia Britannica, Inc., 2024, www.britannica.com/topic/Cologne-Cathedral.

Shelley, Mary. Frankenstein; or, The Modern Prometheus. 1818. Project Gutenberg, 1993.