The City God is, well, a god who watches over a city, town, village, etc. Self-explanatory. There isn't a single city god though (please correct me if I'm wrong), so a city worships a unique deity of its own, either a folk deity or a deified soul of a deceased person who was once in a position of power (the magistrate, governor, etc.).
So, who is Taiwan's City God? No idea!
Looking through the temple's official website, I found this paragraph:
Rough translation with lots of help from hubby:
In the second year of Hongwu, from central to rural areas Ming Taizu (Great Patriarch of Ming, i.e. founder and first emperor of the Ming Dynasty, reign name Hongwu) established the Capital City God as "Shengfu Mingling Wang," Provincial City God as "Weiling Gong," State City God as "Lingyou Hou," County City God as "Xianyou Bo", [and the god for] Townships as "Jingzhu."
Basically, we've got these "titles" for the gods but not really "who" they are. The rest of the page gave a brief of the history of the temple, but again nothing about who the Taiwan City God is or even his/her title. I'm guessing they left out the god's title due to the sensitive nature of politics...
Anyway, may be I haven't been in Taiwan long enough to get used to the sights of temples and shrines there, but so far they've been a massive sensory overload for me, much more so than those in Japan. It's a combination of all the lights, the colors, and especially the strong and overwhelming incense. I've always had a sensitive nose, even before pregnancy, and entering these temples is like walking into a wall, a pretty rude awakening.