At the end of the ‘The Belgian Problem’ mission, you get to choose between two options. You’ll unlock this trophy If you choose to 'Destroy the building’
Before completing this mission, make a manual save. Once you’ve completed this mission, reload that save and replay this mission and choose the different outcome to unlock “Tower Defense”
If you didn’t make a manual save before completing this mission, you’ll have to start a new playthrough. Be careful not to save over your original file!
You can also join a co-op game with someone playing this mission. If he/she chooses the option you haven't unlocked, you'll obtain the trophy as well.