This game is using Flash! After the 31st of July 2019, your internet browser might start blocking Flash, especially if you use Google Chrome. This means you'll have to activate it to play this game.
In the game Metal Slug Death Defence, hold left click and release to aim and fire. Press the button to accumulate energy. You can choose weapons. Press numeric keys 1-5 to switch special arms. Their bullets would be placed on the ground after the cannon hitting. Use space bar to switch normal arms. Shooting oil drums to add health and shooting the weapon boxes to enlarge the power of the normal arm.