sci-fi/ fantasy book with dungeons and magic, a man who gets put in a new world and fights to survive and thrive while helping others

[Jordan]

audiobook about a man (the MC) who gets put in an alternate timeline (I believe, could be alternate dimension) where Rome still lives though one last city and there are dungeons and magic. There are safe towns where the dungeons can't spawn but the hunters need to protect the bridge (in the roman town) from the monsters that show up only at night. The MC goes into dungeons with his pistol and levels up. He finds a young girl in the town and sudo adopts her, he then helps her become a merchant. he builds up a legion of soldiers. he is in conflict with the Ceasar of the last roman city and ends up evacuating with a lot of people from the town to explore the rest of the world. I believe that they get the skills and classes though orbs or shards from the dungeon.

[Kris]

Jordan, around what year did you read this book?

What is the man's job? Does he have special skills?
How did he get transported to this alternate timeline? Was he alone?