The game's difficulty levels are called tiers, ranging from Tier 1 to Tier 18.
You unlock the subsequent tier in Milestones by reaching 100 waves on your current tier (except for at tier 15 and above you will need to reach 300 waves to unlock the next tier).
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PS! The phrase “tier” may also be used in other contexts, such as different “tournament tiers” (e.g., Champion Tournament and Legend Tournament) or to specify the type of merged module (e.g., a mythic tier module, though this is usually instead referred to as Module Rarity). “Tier” is also used in contexts such as “tier lists” that gives a rating to a subject matter (e.g., Lab Tier List).
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