Defend THIS! is the fourth level in Marathon. It is the first level of Counterattack, the second chapter of the game. The three circuit boards that you retrieved in the last level have to be brought to the Defense Control Center at the end of this level.
There are two Pattern Buffers in this level; one near the beginning of the level in an ammunition storage room, and one near the end. Each pattern buffer is accompanied by a 1x shield recharger, and a third 1x shield recharger can be found in a tunnel halfway through the level.
The level contains a mix of wide open hallways and narrow tunnels. It appears to be a militarized defense sector, containing defense drones, an ammunition cache and the Defense Control Center. The first part of the level is focused on combat, being filled with Pfhor Fighters and S'pht Compilers. Fortunately, there are also some friendly defense drones to help you. The second part of the level not only contains aliens but also several button puzzles, where a series of buttons must be pressed in the correct order to proceed.
Move north through the corridors; you will soon find yourself in a large garbage crusher room where the floor will start to rise, threatening to crush you against the ceiling. In the north wall, near the western corner, there is a hidden door that can be opened to escape. At the split in the path, take the left branch to reach a room with a large amount of ammo, a Pattern Buffer, and a 1x shield recharger (room B on the map).
Retrace your steps, making your way south through some radioactive rooms. The southernmost wall here is actually a hidden door, behind which you will find a hidden room with some ammo and a terminal.
Further on in the level you will find a large, dark hall with a bridge leading across; Fighters will attack you from niches up around the room's walls. Shortly afterwards, in a series of tunnels, a 1x shield recharger can be found, as well as a terminal.
Continuing on you will soon find a very large room filled with Fighters, Compilers and robotic defense drones. Don't shoot the drones, they're on your side. Kill the aliens and hit the switch to lower the elevator in the middle of the room, allowing you to continue on.
The next room (room D on the map) contains several buttons. This is a puzzle: find the right buttons to press in order to open the door.
The next room contains another button puzzle, where the correct buttons must be pressed to raise/lower some platforms. Simply press the second button from the left to solve the puzzle. Don't forget to make use of this room's 1x shield recharger and Pattern Buffer.
The fifth and final terminal can be found just beyond. Before Leela allows you to move to the next level, you have to install the circuit chips in three different rooms to the south. Hug the walls to avoid being crushed by the automated crushers. Install the chips in the three small side rooms (be careful, each room is filled with enemies), and return to the fifth terminal. From there you will be teleported to Couch Fishing.
In this terminal Leela mentions the invaders seem to consist of several different species called the Pfhor and the S'pht. She informs you she has established contact with Durandal. He is apparently not as damaged as initially thought and has communicated with the aliens. Leela also tells you where to go to insert the circuits you retrieved in the last level, and she reveals the location of a shield recharger and ammunitions cache.
This terminal is a corrupted message from Tycho saying that Durandal has been rampant for years and that Tycho is being assimilated. There is obviously more to the message, but due to the corruption it cannot be deciphered.
This terminal tells you that there has been a security breach in your location and goes on to quote an article about rampancy in AIs. Afterwards, it will tell you about the breach again before disconnecting.
This terminal also holds a message from Leela. Once the chips are installed, she says that the Marathon Automated Defense Drones have activated and that the Pfhor will encounter stiff resistance. She then tells you that seven Pfhor dropships landed on the colony and that she has lost communications.
- The music for this level is a cheerful song titled Flippant. It is track #3.
Marathon - Level 3: Defend THIS!