In Fortified, Alien has mastered the earth, and brief instructions describe the creation of a secret task by the government in an effort to eradicate them. This take-down is done through the completion of a series of maps, where you are tasked to defend the rockets of alien swarms. After receiving the amount of damages, the ship will fall off base and you will lose.
Each map has a number of waves of this army of alien invaders came in, giving players time between waves to form additional structures to keep the spacecraft, leaving you as – hopefully – the last line of defense for any who were able to sneak past your trap. The amount of time between waves depending on the selected difficulty – normal gives you limited time to sell unnecessary structures and new places, while hard to give 15 seconds and Insane allows only 5. When starting the map, your character is given a set amount in-game cash, which is used for the structure.
Killing the enemy during the wave of increasing cash available, which allows you to add new defenses and upgrades between waves. These resources are limited because of the difficulty in progress, so that planning and strategy is key.