One of the achievements are Twist and Shoutnote This achievement requires Spear Maiden from Level 7 to use their spin attack 50 times.
Similar to the Forgotten Treasures mini campaign in Origins (see the below folder), there are the Guest-Star Party Member dwarven units in The Dark Descent, where the leader is a Composite Character of Durin Longbeard and Gimli, speak dwarvish, have Mithril-based gear, are named after several dwarven characters from Lord of the Rings, and of course, quote lines from the Franchise.
The Flesh Golem that's an ability you can get via The Dark Forge in Bonesburg is named Frankie.
The "God King" in the endless mode Ruins of Nas'de is an obvious homage to Xerxes.
The level Dunes of Despair, features an enormous Sand Worm in a clear reference to Dune.
When attaining or upgrading the Falconer ability, the Crossbow Fort shouts "Blackbird, fly!"
Kingdom Rush: Origins
The entire Forgotten Treasures campaign is a shout out to the fall of Moria in The Lord of the Rings. The Dwarves had Dug Too Deep and uncovered the Balrog Godieth, which easily exterminated them. Godieth's Flavor Text even implies that Durin's Bane is his " 'more famous' big brother".
Also with the Weirdwoods, their name and appearance are very similar to Weirwoods.
Malicia, the Queen of the twilight elves, is an Expy of Maleficient.
A surprisingly well-researched one that borders on Genius Bonus: the achievement for defeating Baj'nimen the Hateful, who rides on an oversized centipede monster, is "My Lord Bag of Rice",
In the level Beheader's Seat, if you click on the bush in the middle of the level, Roadrunner and Wile E. Coyote would appear to do their gag. The tunnel and a set of TNT are also seen on the left side of the level. Also, one of the statue showing a female plump ogre is based on the Venus of Willendorf figures. The level has digsites which wouldn't look out of place in Jurassic Park.
Chomp Bots look very similar to Mousers from the Teenage Mutant Ninja Turtles series, the achievement for beating enough of them is even called Mouserthon.
The names for the Dark Knights are based on actors who have played or voiced Batman in various media before.
The Northern Tribe are essentially vikings with trained dragons, with Jokull being a massive dragon similar to the Red Death or Bewilderbeast. You even get an achievement called "How to tame your dragon" when beating Jokull. The Leap Dragons also resemble something that wouldn't be out of place in How To Train Your Dragon.
One of the tower types appears to be a crypt that can "charge" up to three souls as projectiles, similar to the crypts from Cursed Treasure.
The Doom Tank SG-11 Hero Unit is a direst reference to the Mark X from Ironhide's other game, Iron Marines. It's a huge tank that's capable of summoning soldiers as temporary reinforcements, and also has an attack which covers an area in hazardous stuff that deals Damage Over Time.