WARNING! This post, or any further parts of this feature, could contain spoilers for some who have not played the mission. As such, please be aware of this!
The Lost Levels (full name Descent 1: The Lost Levels) is, in terms of level count, one of the largest Descent (II) missions ever created. Its 25 levels (inclusive of one secret level that takes multiple visits to fully explore) is exceeded, not counting the conversion of D1 or Descent Maximum to D2, only by the 27 levels in Vignettes (which are much smaller on average) and the 32 in The Enemy Within (which again tend to be a bit smaller though bigger than Vignettes). The arrangement of the 24 normal levels is not dissimilar to that in Counerstrike!, but there is a briefing on the story before every level. There are six bosses, one for every four levels. Most of the 4-level "units" themselves are themed, often matching to the themes of the original, though the most prominent textures used to induce the theme can be a bit different. Each normal level also has a Full Map, but regardless of how easy it may be to spot, getting it can be a different story - and oftentimes that applies to secrets in general.
The mission comes with new robots in the form of modified Vertigo bots, and, rarely for a Descent mission, modified weapons as well! The Lasers are the one weapon that is largely unmodified. The Vulcan, now Tri-Shard, Cannon shoots visible shared so you can see where they are going. The Gauss, renamed Vauss, is no longer a rapid fire weapon and takes a lot of ammo to fire off a shot. However, while the shots look like a single Gauss shot, they are VERY powerful. They will kill most enemies with a single hit. The Spreadfire Cannon is now a Missile Launcher and actually fires energy-powered missiles, with splash damage an all else! The Helix Cannon is now an extremely powerful Magma Gun, which shoots out slightly homing magma at a rate that will quickly tear any robot to shreds (except bosses). It isn't a good gun for the long range though, and that isn't its biggest drawback. The rate at which this thing uses energy is literally off the charts - imagine the Helix Cannon multiplied by a factor of four, or even five. Literally it will, on Hotshot, use a fresh 100% of power in a mere five seconds. This is no exaggeration - you can check it out yourself. The Plasma Gun is now a Crystal Launcher and its shots are rather weak but they home leading to it becoming my weapon of choice much of the time. You might be surprised how powerful it is to have homing shots - weak or not. The Phoenix Cannon now fires Smart Mine energy, and is faster and more powerful, but quite energy-expensive. The Fusion Cannon is now the Fusion Core, and the shots look the same, but they home in addition to doing Descent I level damage. But in addition to charging, firing the shot itself now uses up quite a lot of energy, so it has its downside. The Omega Cannon now homes on targets as well, and is otherwise unchanged.
Secondaries are also modified. Concussion, Flash, and Homing Missiles fire faster, and the former two also fire in pairs, though they use two ammo accordingly. Guided Missiles now disperse energy when they explode, in the vein of Smart Missiles, though secret hunters will often find themselves using them for a completely different reason. Proximity Mines now brighten up the area around them. Smart Mines now make larger explosions and also release homing Plasma Balls instead of Plasmatic Energy. Mercury Missiles are now Frag Missiles and while the main shot looks the same, they now explode into a splattering of Concussion Missiles! But you can't fire them as fast, which is the downside. Plasma Missiles, replacing the former Smart Missiles, aren't changed much except in name and that the released energy is now white instead of green. Mega, now Cyclone Missiles, no longer have homing power, but a small change in the look of the projectile (spikes on the sides) means a big change upon explosion. Homing Missiles are released in all directions upon impact, and the splash damage radius of the explosion itself is huge as is always was. And last but not least, there's the mother of all weapons, the Impact Nuke. This slow-moving projectile will actually darken the area underneath it - that should be a hint about its power. And powerful it is, no longer with any "children", but it doesn't need them! Literally the impact of one is like an earthquake, and needless to say chances are that anything that can see it when it explodes will be history in no time flat, excluding only the most powerful of bosses and mini-bosses.
Robots include some from all Descent variations, including Vertigo. Some of them, especially Vertigo ones, are modifies and some can also use the modified weapons themselves. The WASP is possibly the biggest case of this, and its Mercury Missiles are now Frag Missiles that will release their lethal Concussion Missile assaults. You are briefed to each Vertigo robot, but not the largely unmodified Descent I and II robots. And like mentioned before there is a custom boss every four levels, some incredibly lethal. You yourself have a different ship too, and in older versions it was more challenging to control due to a higher speed, though in the latest version that is no more.
Feature-wise, that leaves just the music. There is no original soundtrack, but 24 fitting tunes are used. Most often are Descent I musics, but each boss level has rockin' Final Fantasy 7 music, and some normal levels, generally the later ones, use some interesting different choices as well.
Since just explaining the features of this mission was (at least) a full post length, that's it for Part I. But be sure to return to Part II, which will begin a full level by level analysis! :)