This is the last achievement in this game. Once you complete the mission in Pompeii, you will be brought back to the briefing room. Once here someone will appear and have a conversion with Dr. Koell. During this conversation, make your way to the wooden door located behind the guy talking to Dr. Koell. Walk up to the door and press and hold
. A blue bar will now fill up. Once the conversation is over, the secret mission will start. Once you gain control of your character, make sure you put the difficulty back to easy to make this a little easier. All you have to do is defend the Development Team. After about three or four waves, you'll be asked to go to the cemetery. Look at all the crosses. Once you looked at all of them about 5-6 enemy troops will ambush you. Kill them all and the achievement will unlock.