There are several types of currency and resources in Mad Rocket Fog of War and as you progress in the game you will need a huge amount of each of them.
Gems are the premium-in game currency and is used to speed up the building constructions and upgrades of your base. Gems can be earned in-game by completing achievements or purchased at the in-app shop using real money. Gems can also be used to buy other resources such as Eternium, Gold, and Cubes in the Treasure menu.
Gold is produced at the Residence and stored in Gold Storage, it is mainly used for upgrading troops and the arms in the armoury. It can also be used to remove tiles.
Eternium is produced at the Eternium Drill and it is used when upgrading facilities on your base such as your armoury and HQ. Eternium can also be used at the in-app shop to purchase the 'Basic Chest' which will contain parts that can upgrade arms.
The Eternium you earn by looting and producing is placed in Eternium Storage and can be withdraw whenever you need it. If Eternium Storages are full, any extra Eternium you loot will be lost. When a Eternium Storage is destroyed in a battle, all the Eternium in the storage will be taken by the enemy.
Cube is produced at the Cube Factory which you will be able to unlock when your HQ level reaches 11. Cube is used for constructing and upgrading buildings. In order for you to protect Cubes your Vault should be at least level 8.
Timer is produced at the Time Compressor and is used to reduce the remaining construction time of any building by 1 hour. Timers can be acquired by:
1. Produced at the Time Compressor
2. Daily Login Reward
3. Season Pass missions (best way to collect Timers)