Finding the Indiana Jones guard house key is one of the first challenges you'll face one you're inside the Vatican. Fortunately it's more a tutorial than a real challenge as the key, and the door it unlocks, are more or less next to each other.
There's a good chance you've found other locked gates and keys prior to this, such as a Blackshirt key gate or the Rampart key which you can find long before you actually reach the Ramparts, but the guard house key is the first one that blocks a path. If you want to find it and see what's beyond the door, here's where you need to go.
