The Archer: Guardian Archers for your side, Crossbow Orcs for the enemy.
Arrows on Fire: The Guardian Archer's arrows can be upgraded with burning pitch. Your crossbow can also be upgrade thus, visibly changing glow colorfrom blue to red.
Blade on a Stick: The Bladestaff, the Apprentice's melee weapon. His has two bladed ends, where the old warmage's had only one.
Blood Knight: The Apprentice is having the time of his life slaughtering Orcs.
Boring, but Practical: The tar pit trap is one of the first trap types that the player unlocks. It does no damage, it cannot throw enemies, and its only role is to slow down invaders who walk across it. However, because this causes hordes of foes who charge in to bunch up when they reach it, it serves as a force-multiplier for Herd Hitting Attack spells and traps. As a result, it will be used in almost every level thereafter.
The player can do this to the enemies by means of physics manipulation, such as the wind belt, springboards, and push traps. The enemies make a costly charge through a Death Course, only to get knocked back to the start of it and be forced to run it again!
Cynical Mentor: The dead teacher will berate and belittle The Apprentice every chance he gets, despite The Apprentice being what can only be described as a walking Apocalypse.
Death Course: As a player, you'd better get good at making them or you won't survive long.
Difficulty Spike: Once you get to the Overpass on nightmare the game will stop taking prisoners.
Some might consider nightmare by itself to be this, as you no longer get any set up time before the start or between waves of orcs.
DLC: Two: "Artifacts of Power" adds a host of new weapons, traps and an alternate outfit for the Apprentice, "The Lost Adventures" adds a set of extra levels. One of these is an inverted version of "The Tower" from the main campaign.
Easy Mode Mockery: If you play on Apprentice, you can only get a maximum of two skulls, instead of the five-skull potential on harder difficulties.
Blow You Away: Wind Belt - Fires a gust of wind to knock back multiple enemies, or can be used to grab and hold a single foe. Useful for buying time or for blowing into stage hazards.
Fearless Fool: The player character is described as being the worst kind of student - foolish. Throughout the levels, he constantly shows no fear and continues to taunt both the sorceress and her hordes, despite the fact that the world appears doomed as there are not enough warmages left to stem the tide.
Old Warmage: "Now she's bound the numberless horde to her will and returns to teach the Order harsh lessons in humility and subservience. But she's in for a surprise; I'm reasonably sure my apprentice is unteachable."
In fact, whenever his character appears to be developing, it goes in the same direction on both accounts: his insight and budding wisdom both come with the admission that he's basically just delaying the inevitable conquest of Earth.
Everything's Deader with Zombies: One of the last upgrades available from the Knowledge Weaver causes some orcs to rise up and fight for you when they die.
Fragile Speedster: Kobolds. They have an annoying habit of outrunning your traps but go down from a single crossbow bolt. This can still be damned dangerous, because the traps they dodge take time to reset, letting slower enemies make it through unharmed...
Genre Busting: As stated above, the game doesn't handle or play like a typical Tower Defense game, relying significantly more on you doing damage to the Orcs than your traps. Relying on traps will quickly prove suboptimal. More often than not, traps are simply there to channel, delay, soften up, and thin out the horde for the player to dispatch more easily.
Guide Dang It: Some of the levels have multiple doors, allowing foes the come from multiple directions at once. However, they tend to stagger their entrance, first breaking down one door, then another in a different round, then eventually coming through both at the same time. Rarely does one begin the level with the resources necessary to cover more than one approach, meaning that the player must know which doors disgorge The Horde in what order. The player may have to resort to playing the first few rounds and then reverting just to figure what to cover when. The final level really takes the cake for this, with its four sets of paired doors.
Happy Dance: The better you do, the better dance you'll get!
Heal Thyself: Enemies occasionally drop health potions.
Heavily Armored Mook: Armored ogres. They can take a fair bit of punishment, and will tick a whopping 10 points off your rift's health, versus only 5 from regular ogres.
Idiot Hero: The Apprentice is dumb as a post, relentlessly enthusiastic, and very, very good at his job. He'd be (more) insufferable if he didn't show the capacity to get worried every now and then. And, of course, it's heavily implied that his survival where seemingly all other War Mages have failed is because he is just so stupidly persistent in the face of all odds.
Jack-of-All-Stats: The Knowledge Weaver essentially makes you this, offering general-use upgrades for you, guardians and traps, as opposed to the trap focus of the Steel Weaver and the weapon/spell focus of the Elemental Weaver.
Last of His Kind: As you progress through the campaign, the Apprentice starts to suspect that he may be the only warmage left. The sorceress confirms this, in an attempt to demoralize him. It doesn't work.
Last Stand: The game encourages this by giving the Apprentice a sharp regeneration boost when he is near the Rift. You will live longer, but die fighting the horde there and it's all over.
Laughably Evil: The orcs, obviously. More than occasionally they come with comments that will draw a smile on the player's face. Sometimes they even scream out "MOMMY!".
Life Drain: The DLC Vampiric Gauntlets have this as their primary attack.
Lightning Bruiser: Gnoll Hunters, which specifically hunt you instead of going for the rift.
The Magic Goes Away: Instead of facing a Bolivian Army Ending the Apprentice decides to close the rift, foiling the sorceress's plan but also robbing his world of magic.
Magic Knight: The Apprentice and warmages in general.
Mana Meter: Used to fuel your spells and some weapon attacks. Recharges by itself slowly, but can be sped up by standing near the Rift or refilled instantly from a Mana Well. The last upgrade from the "elemental" weaver allows you to regain a portion of mana from crossbow and bladestaff kills.
Mercy Mode: Fail a normal (warmage) difficulty stage twice in a row and it will ask if you want to play it on easy (Apprentice) mode.
Mind Control: How the Sorceress is uniting and controlling the races of the orcs' world. The orcs have dialogue that indicates it, as well. As you might imagine, this doesn't bode well for her when the Apprentice shuts down all magic by closing the Rifts.
Mighty Glacier: The armored ogres are fairly slow, and take a ton a punishement.
Mood Whiplash: Contrasting the cartoony graphical style and amusing one-liners is a rather grim plot where the heroic faction is slowly getting picked off and pushed back, and is on the brink of invasion by foes whose own world has gone to crap.
Shield Bearing Mook: Later levels give some of the basic orcs shields. They generally only take one hit to destroy, but prevent that first hit from being a headshot.
Spike Balls of Doom: The player can use them in the swinging mace trap, where they are positioned as a weighted pendulum swinging from a ceiling mount. They are arguably one of the highest damage output traps that the player can place, but are only effective if enemies can be kept underneath their arc for long enough for it to swing by, and it can only be placed in very limited areas. It is also the most expensive single trap in the game.
Stone Wall: The Paladins the player can summon. They do pretty modest damage, but they are very tough, and attract any nearby enemies that attack players and guardians. They can be overwhelmed on the front line, but serve admirably as a rear-guard in a tightly packed space. The Steel Weaver can upgrade them to regenerate health and stun enemies with their sword, turning them from rear-guards to frontline shock-troops.
Stuck Items: The crossbow is always in the first spell slot.
Shout Out: At the end of a wave, the Apprentice may invoke Charlie Sheen's "Winning!" line.
Smash Mook: The Ogres can take a lot of punishment, and also cause much more damage if they should reach your rift. They aren't affected by physics-based traps unless you upgrade them specifically to do so.
Springs Springs Everywhere: Spring Traps. Though, these affect the enemies rather then you, and always launch them in a specific direction, which you set when first placing them.
Stripped to the Bone: The acid bombs from the DLC Alchemist's Satchel inflict this on enemies, as do the automatically-refilling vats of acid in the Lost Adventures DLC.
This Loser Is You: The game starts with The Apprentice's master dying by cracking his head open on a rift staircase. As he's dying he reflects on his awesome Orc slaughtering accomplishments and goes on to say hope is lost now that it's up to The Apprentice given his rather low opinion of him.
Too Dumb to Live: The traps aren't exactly hidden or anything. The Orcs will blindly run into them anyway. Good for you.
Some of the comments the orcs make about the sorceress in their head suggests at least part of this is her forcing them onward.
Hilariously, the orcs often remind each to watch out for traps, and then proceed walking onto said traps themselves, along with the ones who were warned.
Vent Physics: Vent Traps launch enemies up in the air, opening them up to further attacks.
The Un Fought: You never actually fight the sorceress, it is heavily implied at the end she dies at the hands of the orcs after losing her magical control over them.
We Need a Distraction: Orcs will stop to attack bomb barrels, Decoy traps and guardians. Paladins are especially good at this, tanking groups of Orcs while you hit them with an Area of Effect attack. Kobolds will run right past them.