In Doom: The Dark Ages' sixth mission, Siege Part 1, you'll run into multiple secrets that require the purple key. Most of the time, the keys and keycards you need are close to the secret you're trying to unlock, but that's not the case here.
Since the map is massive and multiple Doom: The Dark Ages Siege Part 1 secrets require the purple key, it can be quite a trek, especially if you don't know where to look. To save yourself some trouble and get back into the action quicker, here's where you'll find the Siege Part 1 secret key.
Doom: The Dark Ages purple secret key location in Siege Part 1
The purple key in this mission is hidden at a small outpost in the centre of the map, near a cave. It's a short walk north from the Sentinel Shrine where you first enter the area. You'll know you're in the right place when you encounter a fight with two mancubus demons near some stalls. The key is just up the hill to the left on a little overlook.
Once you've grabbed the key, you can use it to open any of the locked doors with the matching icon. In fact, there's a cave right next to the key which requires you to have the key to access it, giving you the Slayer toy for your troubles. A few other secret areas in Siege Part 1 need the key, but they're much further away, such as the door blocking the Impaler Nightmare weapon skin in a cave to the far left of the map.
