- Ships are modeled as a series of rooms connected by doors.
- Crew occupy one space in a room, move at some set speed, and their actions are determined by the room they occupy. (A crew member in the cockpit is a pilot, etc.)
- Some rooms have a "system" within them, and some do not.
- Each room has an Oxygen level, displayed visually by the color of the room.
- Weapon hits affect a single room - dividing their damage between the hull, systems, and crew.
- Each ship has some number of "power" cells, which may be allocated between systems.
- Some systems (shields, weapons, drones) have sub-elements which divide up the power provided to their parent system.
When you fight another ship (fighting other ships is about 90% of what you'll do in the game) they fire weapons at you - these weapons might hit your shields, or they might hit a room in your ship. If they hit a room they damage your hull, systems in that room, and any folks in that room. Some weapons can even cause a breach - creating a hole in the room which quickly sucks out all the oxygen.
If you have a breach it is important to close all the doors around that room, so that most of your ship still has oxygen. Then again, if a fire is started on your ship you might find that venting all the oxygen out of the room is the fastest & easiest way to put it out. Now imagine what happens if your "Door" system is damaged such that you can no longer remotely open & close doors - you might be in a situation where you are frantically trying to repair the door system as the oxygen drains out of the room.
And all the meanwhile that enemy ship continues to fire on you, and you continue to fire on it. If your oxygen system breaks it's not too bad - assuming that you aren't fighting breaches or fires!
The best defense
Initially players of FTL will focus on keeping their own ship alive. But eventually you realize that you can use your knowledge of the simulation elements to defeat your enemies more rapidly. You can upgrade your surveillance systems (or use psychic crew members) to see where the crew is working on your opponent's ship - a few firebombs in those systems might send them scrambling, unable to man the weapons. Boarding actions are very exciting - you can send a crack team into the enemy ship to wreak havoc on their systems even without punching through their shields. But do they have upgraded security doors that will slow down your assault?
Engine upgrades, pilot skill, and engineering skill all contribute to your ship's evasion chance - by focusing on engine power you can dodge a lot of attacks! But without a pilot your evasion drops to 0. This makes attacks on the enemy pilot quite attractive, especially against faster ships.
The final piece of the sim is your power allocation. This is pretty much the same allocation problem we used in Tie Fighter (or Cybercon III!). Put all your power into the engines to boost your evasion rate. Power up shields to prevent damage. Allocate more power to weapons to keep more turrets firing. Don't forget to keep power running to your oxygen and med bay!
The really interesting, sensible twist here is that you cannot simply put an arbitrary amount of power into any system - each system is limited by a power cell ranking, which you have to increase as an upgrade. So you purchase increases to each system's power limit separately from simply increasing the maximum number of power cells you can deploy. The pricing scheme really encourages you to purchase increased limits before purchasing more power - which ensures that you always have interesting decisions to make regarding power allocation.
These power cell units are ALSO your systems HP - so adding more power capacity to weapon means it is harder for opponents to take out your weapon systems. It is a wonderfully elegant system. This also means that when a system is temporarily damaged, you gain unallocated power which can be put elsewhere. I'll often increase my engine power when my shields or weapons are down.
All in All
Is FTL a great game? Maybe not. It has a pretty limited scope, which gives it a very niche appeal. But it fills that niche very well. It's a great example of making a compelling game for a limited audience, and not charging an arm and a leg for it. I think that's exactly the direction developers should be moving in, especially for premium games.