There are six tiers of turrets: D, C, B, A, S, and SS. You can only build D-tier turrets, and two identical turrets can merge into a higher-tier turret.
You can upgrade your turrets by race. The upgrade will affect all turrets of that race.
Upgrade consumes energy balls. You can activate and upgrade the energy ball generator to collect them.
Killing bosses can let you gather more game resources. You have the control of when and which boss to release.
You can upgrade S turrets into SS turrets and choose which one to upgrade.
SS Turrets have special skills. You can upgrade a turret to SS-tier to obtain the corresponding skill.
You can obtain extra game resources by completing various tasks, such as owning combinations of specific turrets.
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