The Indiana Jones guard house key is one of the first to find on the way to the Vatican. It's more of a tutorial though than a real challenge, as a result because the key and the door it unlocks are more or less next to each other. There are multiple routes around and through the section it appears in meaning you don't even need to find it to progress.
At this point you'll probably have encountered other locked gates and keys, with the Indiana Jones Blackshirt key the first you'll see mentioned. You won't get near that for a while, while the a Rampart key you can find near is actually used to open a shortcut after you find it.
However, while all these introduce keys in general, the guard house key is the first one that appears to actively block your way ahead until you find it. If you want to get it and see what's beyond the door it locks, here's where you need to go.
Indiana Jones Guard House key location
The guard house key in Indiana Jones opens this door on your way to the Vatican, which states it 'Requires: Guard House Key' when approach it. Fortunately, it's not far away- if you just keep going past the door you'll see this little room on the left, just as the 'Bastione San Giovanni' message pops:
All you need to do to get the key then is sneak in here, deal with the Blackshirt inside, and you'll find the key hanging on the noticeboard. Specifically, the guard house key is here on the map:
If you use the key back at the door you'll be able to go up some stairs and open up a couple more routes to the way ahead - there's some double doors you can also open with the key again to get out, or a ladder you can take. It all heads the same way though, as this part is fairly linear but it gives you some choices and more things to explore.
You can still get where you need to go by just walking straight on and never touching the key or the door it opens. If you do that then those double doors out I mentioned might be where you first discover the need for the key. If that's the case then it will be a little further away to backtrack to it but you can still grab it in the same place.
However you use it, finding and using it basically just give you some different paths to play with as you explore Indiana Jones and the Great Circle.
If you're thinking about more adventure once Indy's done then check out the Best adventure games.
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