Descent II focuses on systems beyond the solar system. The planets are Zeta Aquilae, Quartzon, Brimspark, Limefrost Spiral, Baloris Prime, and Omega. The Omega system is subdivided into the Puuma Sphere and Tycho Brahe, with the latter being the final level of the game. Each system consists of 4 regular levels and 1 secret level.
In response to complaints that Descent's levels were mostly dull and lacked creativity, Descent II's levels were designed with a theme in mind; as an example, Level 2 "Turnabout Bore" lives up to its name since the map resembles a figure-8. There is the inclusion of difficult puzzles such as hidden doors and laser-reflecting force fields; mostly to hide valuable powerups or hostages, but some are required to complete the level.
The secret levels are unique in that the player can travel back and forth between the regular levels (of the same system) via teleporters so long as the reactor of the secret level is not destroyed. However the player may not save nor open a game on the secret level, since it would take the fun out of getting through single-open doors and hitting switches correctly the first time.
Some of these planets have notable characteristics:
Baloris Prime is mostly desert. According to the writers, this is because its axis of rotation is exactly perpendicular to its plane of orbit, causing a total lack of seasons on the planet's surface. System´s name is pun on the word "bolaris"
Tycho Brahe is actually a spaceship the size and shape of a planet, easily mistaken for one until its two hemispheres separate to reveal a mechanical interior.
Brimspark is pun on the Otto von Bismarck
Ahayweh Gate, the first level, is named for the phrase "Abandon hope, all ye who enter here" from Dante's Divine Comedy.
Gytowt Station— The first word of this level name could be read in a similar way to the phrase "Get out," perhaps referring to the proliferation of locked doors and walled-off paths in the level.
Galacia Caverns— The fusion reactor in this level is located very close to the beginning, but the self-destruct sequence which starts when it is destroyed is extremely long compared to a normal self-destruct countdown.
Chain Reaction— This, the final secret level in the entire game, has the distinction of being the only level with multiple fusion reactors. There is one immediately at the beginning of the secret level, and one in each of many alcoves within the level. After each additional reactor is destroyed, time is added onto the self-destruct sequence.
The complete list of levels from the original Descent II mission is as follows:
Level 1: Ahayweh Gate
Level 2: Turnabout Bore
Level 3: Wenl Mine
Level 4: Robby Station
Secret Level 1: Segment City
Level 5: Seaspring Gorge
Level 6: The Well
Level 7: Coralbank Quarry
Level 8: Riverbed Mine
Secret Level 2: Hurdle Chase
Level 9: Firewalker Mine
Level 10: Lavafalls Extraction Ctr.
Level 11: Coalbank Shaft
Level 12: Magnacore Station
Secret Level 3: Mephisto Hardcore
Level 13: Sleetstone Tunnels
Level 14: Arcticon Corridor
Level 15: Icehammer Caverns
Level 16: Terrafrost Catacombs
Secret Level 4: Galacia Caverns
Level 17: Y'tor III
Level 18: Drec'nilbie K'luh
Level 19: Nep-Hilim S'crub
Level 20: Gytowt Station
Secret Level 5: Ascent Level 1
Level 21: N'neri Ring
Level 22: Kwod A'rior
Level 23: Iwihml
Level 24: Tycho Brahe
Secret Level 6: Chain Reaction
Planets come with a complementary set of "themed" robots, instead of recycling enemies like Doom. For example, Brimspark (a volcanic lava planet) bots fire yellow/orange blast or explosive weapons, while in Limefrost Spiral (an ice world) bots unleash blue/white bursts from their Spreadfire and Helix cannons.
There are other robots that appear regularly in the game regardless of which planet they are in. One interesting enemy is the Diamond Claw, which is similar to the green Medium Lifters with bloodied claws from the original Descent, but the Diamond Claws can also "short-circuit" and release homing plasmas when struck with any energy weapon (i.e. any laser but not Vulcan, Gauss, mines or missiles), presenting an additional challenge to the player.
Descent II made major improvements in AI, with robots being able to do hit-and-run attacks or roaming through the level, such as the ITD "Eagle Bot" and ITSC. Most infamous was the Bandit or Thief-Bot which was a fast-moving and hard-to-kill enemy that attempted to steal the player's weapons and equipment; the similar E-Bandit will drain the player's energy and shields.
Another notable addition to the game was the Guide-Bot, a companion robot the player could use to aid in navigation and other tasks. It shot flares which could slightly damage the player or enemies, and was immune to laser fire but could be killed by a large amount of missile or Gauss splash damage from either the player or enemy robots.
Like Descent, some levels replace the reactor with a boss robot that must be destroyed to trigger the escape. Descent II places a boss on the fourth level of each system, giving a better thrill throughout the game. Like Descent, all bosses are capable of 'cloaked teleporting' and spawning robots when attacked.
Descent II bosses add a new feature: the ability to generate homing plasma pulses when hit by energy weapons (much like those released by a Smart Missile, but weaker).
The first two bosses are vulnerable to all weapons, while the next two bosses are particularly vulnerable to particle weapons (i.e. Vulcan and Gauss Cannons, and missiles) but invulnerable to energy weapons. The fifth boss can only be damaged by energy weapons, while the final boss can only be killed by hitting a glowing green spot on its back.
Their traits are listed as follow:
Zeta Aquilae System: Nicknamed the "Red Fatty Boss", this boss carries three missile launchers on its underside, which are capable of firing 2 flash missiles and 1 homing missile spontaneously. In the game, the boss is only 'released' when the player reaches the centre of the boss room, unlocking a hidden room. The polygon model for this boss was later reused for the "Red Fatty Jr."/"Tiger" mini-boss (armed with Phoenix Cannon and Mercury Missiles) in Brimspark.
Quartzon System: The "Water Boss" lays Smart Mines around its territory and fires multiple mercury missiles at intruders. The hunch here is the limited space in the room makes it difficult to evade the missiles. It also makes the strangest robot sounds of all robots in the game.
Brimspark System: The "Fire Boss" has a Mega Missile launcher on one arm and an extra-rapid Phoenix Cannon on the other. Considered significantly more dangerous than earlier bosses, the open grounds makes avoiding its Mega Missiles a challenge (both the warhead itself and also its large blast radius).
Limefrost Spiral System: The "Ice Boss" is possibly the next toughest boss in Descent II after the final boss and perhaps the most difficult to confront. It has an Omega Cannon on one arm and a homing flash missile system on the other. The homing flash missiles are not only incredibly tough to evade, but also has a large blast radius and has a more powerful effect than the usual ones; a single hit is able to keep the player under constant blindness.
Baloris Prime System: The "Alien 1 Boss" fires Gauss Cannon and Mercury Missiles. Its appearance is considered by players to be the coolest in the game. This boss is not too challenging, except for the fact that it is only vulnerable to energy-based weapons (though Parallax included Earthshakers into 'energy weapons' also).
Puuma Sphere / Tycho Brahe System: The "Alien 2 Boss" requires the most skill to defeat of all. It can fire multiple Earthshaker Missiles, whose damage is not only fatal but also disorients any navigation even if you did not receive a direct impact. It is designed in a way that it can be only damaged by hitting a spot on its back; this gained some infamy when a bug (subsequently fixed by a patch) made the boss completely invincible at higher difficulty levels. Tactics to combat the Alien 2 Boss include using the Smart or Earthshaker Missiles or the Phoenix cannon, all of which can bounce off of a wall to hit the boss from behind. A cloak powerup will also allow the player to sneak the boss and attack his behind.
Saving and restoring can be done at any time, except in the secret levels as mentioned above; it stores the exact details of every object's location and data within the level, and a small snapshot of the player's view, all at the time of the save.