Income and cash flow Edit
One of the main struggles in Return of the Titans is earning money. Towers and other defenses cost money, and you may quickly drain your money if you don't take special care. There are five ways to gain enough money to overcome the titans:
Crystal Refining Edit
Beginning with the second mission, you will be able to build a Refinery. You can use refineries to refine money from crystal deposits on the map. Refining crystals is the main source of income, and you should aim to build two to four refinery on every crystal. Of course, titans' spawn points may complicate refinery placement.
At the end of the level, you gain 10% of the unmined crystal.
The passives mining boost, Extraction Tuning and Extraction Calibration, can be useful, but require quite a bit of prerequesite. They also increase the crystal scavenging rate from 10% to 15% then 20%
All in all, the refinery is the only mandatory building to finish the game. Further development can be useful but is really up to personal playstyle.
A lot of medal can award you money. Most are one-time only, but some can be earned on every missions : Pristine, Careful ($250), Pristine ($500), and Efficient ($250). All are reward for goal you should already aim at, and they are nothing to scoff at, especially in the early game.
Building recycling Edit
40% of your batiment cost is refunded when you sell the batiment or when the level end. That's why you should as much as possible try to avoid tower destruction. Selling disused towers and refinery in difficult level can also be a smart move.
That's also why really expensive buildings like heavy weapon, factory and especially Tank Factory should never be allowed to be lost.
Boss kill Edit
All boss award an hefty amount of money ($1000 time the world the boss first appear). That's mainly handy on the last world, where the game start to send bosses against you every other level.