Between the 20th December and the 9th January of each year, the game unlocks "Yeti mode" - in addition to a winter-themed introduction screen.
Yeti Attack! Edit
The goal is to survive for 30 minutes against the waves of Yeti Titans. They want to cook the poor humans in order to serve them for some strange alien holiday, so don't let them!
You have every standard building available, and one of each of the limited buildings (one mission's worth of production to be precise, so 1 laser, 40 concrete wall, etc...)
The map is random, with only one Titan spawn point and one path to your base. A lot of crystal will appear at first, but not a lot will spawn later on. This setup makes it a lot easier to build an efficient defense than in survival mode, and it does look like a lot to more traditional tower defense.
Game Progression Edit
The game begins very slowly at first, with a lot of crystal to harvest and very weak Titans, and the difficulty ramps up more and more quickly. A number of Mega Yeti (four or five) will also attack your base; they work like the MegaTitan, the first earth boss.
The regular Yeti work like Titans found on Earth and the Moon, and do resemble the regular Titans with a lot of wool tacked on. You will encounter only a few armored Titans, and mainly at the end, no Ghost, no Titan hatchlings, no Armored Titans, and seemingly no more than 8 armor rating.
However, they do gain more and more life, and some of them are Aggressive, so the end of the mode can be quite stressfull. You should try to keep your powerups as long as possible, to deal with the last waves.
You have plenty of time to harvest crystal, so don't overdose on refineries and if an area allows the harvest of more than 3 or 4 crystal use Reprocessors to make more money.
Limited-production constructions are very precious, so try your best not to let them be destroyed !