Army of Ages
Here's a short synopsis of Charles Darwin's Origin of the Species, in case you don't have six dollars for the Cliff's Notes: Adapt or die. Now, that's all well and good for Dodo Birds and Humpback Whales, you might say, but what does that have to do with me? In Louissi and Mapoga's polished strategy game, Army of Ages, you begin as a lowly stone age clan when advanced alien insectoids invade your world. You'll have to evolve through five ages before you finally have the strength to take down the hive.
Each stage in your evolution will offer you first and second tier
build paths. To make money for units, upgrades, and turrets, you'll need to kill aliens and collect water. To collect water you need to build gatherers. There are only three wells you can gather from and the farther the well from your base, the more money you get for the water. But, if you can't push the alien horde far enough from the water source, your gatherers will be slaughtered. Each time you kill an alien invader you gain experience points. Build up enough XP and you can call on special magic attacks, and, most importantly, evolve to the next stage, which will open up stronger units and turrets to bring victory.
Army of Ages is built upon a great story and features a surprising amount of unit variety (Dragons!) and well balanced upgrade costs for a 30 minute flash game, but the game does have one annoying feature which can make the game seem impossible on harder settings: when you evolve, all your units and turrets disappear, but the aliens who were just attacking you don't, and you will have to quickly rebuild your army before they reach your base. Tip: right before you evolve is a great time to cash in on your magic attacks to clear the map and give yourself some breathing room to rebuild.
That being said, Army of Ages is one of those games that you almost feel guilty playing for free, since it's so well done. But there is always a flip side to a good game: it has to end sometime, and 30 minutes isn't enough time to do this idea justice.