Fortress World
A Fortress World is a classification of planet from the Warhammer 40,000 setting and other sci fi universes. They are categorized by being, essentially, a planet sized military bunker with 24/7 martial law. The most famous of these is the planet of Cadia. Fortress Worlds are one of the lesser common types of planet in the Imperium of Man but may just be among the most important. Were it not for these planets, Abbadon the Despoiler would have likely succeeded in his Black Crusades (but probably still failed miserably anyway), the Tyranids would have nommed through the Eastern Fringe of the galaxy by now, and some Ork WAAAGH!s simply would not have been stopped before they got out of control.
Think a giant stop sign with a motion sensor and machine gun attached to it. It's also mass producing more of such stop signs.
Features of these worlds cities include city sized walls around continent sized cities, every piece of architecture (and I do mean every) being designed with defence above comfort for its citizens in mind, and resource stripped landscapes that make salt flats look like bountiful gardens. The walls are almost always designed to be somewhat hollow with several levels of staircases and landings with one entrance and exit: at the bottom from the outside, and top going in. This makes it INCREDIBLY difficult to take control of the wall by force, as you are always fighting uphill and being shot at from above. On top of that the staircases are rigged to explode and collapse at the push of a button at the top if any are captured.
If, by some miracle, you have taken the outer wall, there are about a dozen more, each taller than the previous so they're all shooting at you. The actual habitation sections of these cities are no more welcoming. The citizens don't live in housing complexes as more giant barracks riddled with heavy weapons emplacements and turrets. Note: Every citizen on a Fortress World is required to be able to man such emplacements as soon as they are able, so it's likely parents are taking their children to target practice on the turrets in between boot camp training sessions where they would have done so already. The streets of the cities are a maze of tactically advantageous corners, barricades, battlements, and firing points that are all capable of being advanced on or abandoned at a moments notice. Also, like the walls, every hab-block is rigged to blow from Control further behind the frontline/market district/downtown/wherever you are at the moment.
But that's just the habitation part of the city! Further behind is the main military section of the city. Here, if you somehow survived this far, you will find the dedicated military of the city. This includes, but is NEVER limited to, anti-air flak turrets, miles of firing lines, countless tank formations, surface-to-orbit artillery, and basically every motherfucker with a gun and a stern expression. Here would also be the factories that produced the weapons, equipment, and ammunition that the planet could do itself without the Adeptus Mechanicus. This means that the defending armies would almost never fall to a war of attrition. The geometry of this part of the city is characterized by a relatively gridded street layout lined with the same barricades and heavy weapons emplacements as the habitation section. The streets themselves are incredibly wide to allow tanks passage and crossfire from squads on either side, so you can shove your cover save up your ass for all the Imperium cares!
At the centre of the city (if you STILL haven't been killed yet) is the Central Spire. This is where the Planetary Governor lives and tells people what to shoot at, while taking aim through a sniper rifle and putting holes in heads every so often. This section of the city is also where the longer range guns that fire at you as you cross the flat barren landscapes before you even made it to the outermost wall. You may also find that this part of the city is almost inaccessible without any aircraft, as this is the most well fortified area in the city. IF you manage to get passed the guarding force of the last HALF of the forces in the city, you still have dozens of meters of concrete, steel, and adamantine to get through. Even if you got through all of the above, the base of the Central Spire is really just the basement; you're still going to have to fight your way up to get to anything useful.
So, after fighting your way across a continent of inhospitable wastes, over a dozen miles high walls (which are fortresses in their own right), through the most grimdark shortcut to the market ever, through the Siege of Berlin on a planetary scale twice, and up a giant fucking tower designed to impede any means of practical movement while being shot at some more, you still have nine or so more cities to capture before the Fortress World is yours.
Oh, before I forget, you're within shooting range of at least three of them at anytime anywhere on the planet.
Good luck!
How To Crack One
A fortress world is one hell of a defensive position. However, they are not invulnerable. To defeat it, assuming you've dealt with the surface to space weapons and enemy navied as well as Orbital forts... You'll to orbitally bombard it, presumably with OKKVs, (Orbital Kinetic Kill Vehicles, aka Rods from God) nuclear bombs of normal and shaped charge variety, shaped charge preferred, and aim for the shield generation systems to soften the area up. Once the majority are gone, try to aim for long range emplacements, in order to minimize attrition. If you're of the chaos type, do try to get cults sabotaging and initiating uprisings. If you're Imperial, Vanus will do the same. Toss in some assassins to remove enemy leadership. Ok, you've successfully blown to bits the shields, and are now constantly raining hell onto the enemy. Time to start the invasion. Bring the Siege Tanks out, and push in one or two, maybe three if you have a huge force, points. At this point, it'd be wise to bombard the flak and other AA batteries. Once they are semi clear, deploy air units to put more holes in the enemy. Congrats, you just broke one of the walls. Now, while this is happening, begin bombardment of other cities and destruction of relief convoys from orbit. Don't let them send reinforcements if you can help it! The other cities should ideally be busy repairing the damage rather than sending artillery to your forces. Meanwhile, back at the coty you're cutting up, use OKKVs to tear down wall sections and cleanly remove resistance. Orbitally drop units in pods or use Elysians and Harakoni Warhawk units. Depends on your allegiance of course. Use them behind lines far enough that you will be able to reinforce them and relieve them. Repeat for each layer of defense, hopefully with some variation to keep them from adapting. Do this to each City. Congratulations, you just expended 10 planets worth of materiel to destroy one fortress world. This, of course, assumes you're not willing to Exterminatus this place. Exterminatus should be carried out via Asteroid or atmosphere burning. This should put a damper on their units. Go in and mop up. Done. As for taking one over, well, unless you have the memetic and psychic corruption agents of Chaos or the sheer idiotic love for the figthing itself of the Orks and a stupid amount manpower on top of that, good luck.
List of Fortress Worlds
Cadia: Destroyed.
Vigilus: In Progress.
Mars: Active.
Holy Terra: Active.
Luna: Active.
Terranis: Destroyed.
Krieg: Active.