Spend money to always upgrade your gun in order to qualify for upcoming missions. Completing the missions more efficiently rewards you with more money. The soldiers that defend your base also generate gold, the game's premium currency. The higher level of those soldiers, the more gold they will bring in. However, they also require gold in order to be upgraded. However, this is worth it over time, as you will eventually recoup the cost of their upgrades. Remember to collect gold from them regularly, as there is a limit that they will hold. Play daily to earn rewards that can include gold.