This Serious Sam 4 Guide and Walkthrough will help you through the mission When in Rome with all side missions.
The level starts with a cutscene which shows Father and Carter trapped, but not injured. Father lied!
Make your way through the sewer. You will encounter a few small enemy spiders, nothing really important.
Continue walking in the sewer until you come across the end of the sewer tunnel. Now you can finally exit this dark place.
After the talk with Father ended, you will see your objective.
Get to the Vatican Library
No, you cannot get to the other side of the river, there’s an invisible wall there.
Since you’re in the open now, you better look out for enemies. There are 2 snipers in front of you on the bridge. Better take care of them quickly, because they actually deal a lot of damage!
Also, Processed are coming your way. If you’ve taken care of them, you have peace… But only for a short amount of time. Go under the bridge and more enemies will spawn, including Gnaars and Drones.
If you walk till the very end of the sidewalk of the river, you will see a small 1HP. Pick it up, you’ll need every bit of health! Suddenly you hear Kamikaze screaming. Turn around and take care of them, before they take care of you.
1HP Kamikaze Trap
You can also just jump into the water, and they cannot get you. Now get up the stairs and there you can pick up some ammo and also get your first side mission in this level.
On the left side there is a person standing, go to him to activate the side mission.
The old guy says something about his daughter requiring help. And because you’re such a gentleman, you will help!
To find Gabriella, just follow the blue waypoint on the left. But be freaking careful, there are enemies out there!
After finishing them off, go left to enter a new area, where you will find a witch bride attacking you.
There are also some other enemies who want to eat your flesh. At some location in this area you can enter a warehouse.
Also make sure to look around you, because there is a room where you can read a letter.