Eliminate the invading force intent on taking Space Station Xorbe in Earth's orbit. If the station is captured then it'll become a staging ground for the invasion of Earth itself. You got a huge arsenal of weapons and defensive mechanisms to stop them.
Place towers on the way of minions of evil pirate who wants to destroy your cake factory. Add special ingredients to towers and make unique recipes that will help you eliminate your enemies.
Will you survive 75 days? Set up a strategy to develop agriculture, handle transactions of agribusiness, trade for guns, train the troops, and send commanders and troops to the battle against the attackers.