Era

Spoiler warning! This article contains spoilers about Era content.

Era is a phase of the game which opens up at Military Tier 8. Era begins when the player reaches wave 100B in endless mode on any difficulty of a given region.

Upon reaching Era, enemies gain an Era shield and various Era powers, depending on their element.

Every time you hit wave 100b, the era counter increases by 1. For example, if you are in era 5 and hit wave 100b, your era increases to 6.

There are several ways to combat the era enemies' shield and various powers, which most are found in Workshop-->Stairs

Here is a general overview of Era workshop. INCLUDES SPOILERS!(credit to @TECHTERROR731 on the discord)

So, Era primer :

-All enemy types have each two new special traits, one offensive and one defensive. -By clicking the stairs icon in the Workshop, you can access the era workshop. Build around either being able to survive hits from an enemy or killing one (you need to be dealt at least 0.1 hp of damage for it to count toward hits taken, so universal shield and way too much elemental resistance won't do the trick). -All enemies have an "era shield" which divides damage taken and increases their attack by 1e20 (multiplied by 100,000,000,000,000,000,000). This shield can be reduced by unlocking the second tab in the era workshop by unlocking both enemy infos of any specific element. This "era shield" increases by a factor of ~50-60 each era.

-Era research (second tab) : Here you can reduce the Era shield using blue resources and blue gems! The first time buying them will reduce their power by 1e20, and each subsequent one will reduce the power of the shield by about how much it would increase by incrementing the era counter by 1. Spending blue resources (and hits taken/kills of the specific element you're researching), you can unlock the ability to disable that enemy's era abilities during runs! Beware that these costs are multiplicative (at a cost of 1000 XP, the second module to disable would cost 1,000,000 XP and the third 1,000,000,000 XP) so choose carefully which one(s) you want to disable each run. Researching the same element 5 times, or bringing the exp cost down to 950xp unlocks the third tab.

-Era experiments (third tab) : Using the same system of kills/hits taken and blue resources, you can gain essence for each element. These essences can then be spent to gamble for special modules, loosely based off their specific element. You need 5 research of an element, or decrease its disable cost down to 950xp on the second tab to do experiments on it. Here, you can also decrease the XP cost of the in-run Era shield reductions using resources. Killing enemies that have their era traits disabled has a chance to drop an artifact that can help with the it.

Tab 1: Era Information

In the first tab of the stairs, you will find various bars, 2 for each element to fill. In order to fill these bars, you must kill era enemies of that element and get hit by them (You must take at least 0.1 damage to count as a hit) during tower testing. Once a bar is filled for an element, you can reveal it to find out one of the element's era powers. When both bars are revealed for one element, that element may now be disabled and will appear in tab 2 and unlock tab 2 if you haven't already done so. You may need to reach era in different regions if you want to reveal all the elements.

Tab 2: Era Research

In the second tab of the stairs, you will find many bars and buttons. At the top, you will see your era dividers. These divide the era shield of era enemies so it is easier to destroy and survive them. You can increase your dividers by pressing the two buttons beside each icon. Under your era dividers you will find a variety of bars and buttons. These let you decrease the cost to disable the powers of each element. The two buttons over the bar will let you speed up the process, at the cost of some hits taken and enemies destroyed. Decreasing the cost to disable the enemy's era powers down to 950xp will make that element appear in tab 3 and unlock tab 3 if you haven't already done so.

Tab 3: Era Experiments

In the final tab of the stairs, there is a bar and many tabs. The bar will let you decrease the cost to temporarily increase your dividers while in a round of the tower testing. You can also speed up the process by using gems once you get the Chained Mannequin or you can speed up it with the power plant once you get the Ominous Coil artifact. Doing an upgrade will upgrade one of the two dividers randomly, except eventually you will want to fully upgrade this so the randomness doesn't really matter.

Formulas, where lvl is the number of already purchased upgrades:

• Upgrade cost is 1.2lvl * 1e15 blue resources.
• The cooldown timer after an upgrade lasts 900 - 3.75 * lvl seconds.
• The cost to skip the cooldown is 1.08lvl * 1000 gems.
• The initial (displayed) XP cost for a divisor is ceiling(0.95lvl * 1e5).
• Upgrade lvl maxes at 240 (120 upgrades for each, capping at 213 XP).

Under the bar there are many tabs. These let you gain elemental essence which can unlock modules to the right of the bar. Once you get to Military Tier 9, elemental essence will also give you a damage boost for each essence you have.

Era Divisors

The divisor buttons are at the top, above the disables. You can also see the effect of the era multipliers in the enemy stats box.

The Era divisors reduce (divide) the effect of the Era multipliers, which (besides the Era Powers) are one of the biggest obstacles in Era. With each subsequent Era, both enemy damage and enemy HP are multiplied by an increasing value. (FIXME: Put exact formula here.) After a few hundred Eras, the multipliers will exceed 1.8e308 (the largest double value) and become infinite, which is represented by a skull and cannot be reduced to a survivable amount by normal means.

To combat this, you can invest XP in the Era Divisors, which divide the multipliers by 8 for each level. Once the divisor amount exceeds 1.8e308, it becomes infinite as well, and (due to game coding) this infinity takes precedence, completely superseding the Era multipliers for the rest of the run and clamping them to their minimum value of 1x. This is sometimes known as "disabling the Era Shield."

However, the initial cost of the divisors is very high - 100000 XP. To be practical, you must use the button in tab 3 in the workshop to reduce the XP cost. Each upgrade multiplies the cost by 0.95, and which one is chosen (HP or damage) is random, but it doesn't matter because you'll want to max them both. Once you've upgraded both 120 times, the cost function will be ceiling((100000 + 10000 * num_upgrades) * 0.95120) = 213 XP initially, growing by ~21.2 each upgrade. This reduces the XP cost to max a divisor to (at most) (342 * 100000 + 342 * 341 * 10000 / 2) * 0.95120 + 342 = 1310537 XP, and less if the tab 2 upgrades have been bought.

Era Powers

Every enemy has certain powers based on the element it has. These powers can be disabled by using the new Era tab during Tower Testing, at the cost of Tower Testing XP. The formula for the XP cost of disables is base_cost * (100 + 0.9 * base_cost)disables, where base_cost is the initial cost (starts at 1000, but can be reduced in Tab 2), and disables is the number of already-disabled powers.

Note that the costs of already-disabled powers don't matter, so it can help to save your lowest-cost disable for last.

The enemy powers are as follows:

Era Powers

Power	Neutral	Fire
Kills	Deals 25% additional random-element damage.	Ignores shield.
Hits	Non-neutral tower damage is reduce by 95%.	Explodes on death.
Power	Water	Nature
Kills	Slows the tower by 250%.	100% Lifesteal.
Hits	80% chance to reduce incoming damage to 10% of MAX. HP.	+25% HP/sec. & +5% MAX. HP/sec.
Power	Earth	Electricity
Kills	Chance to stun the tower.	Warp / Dash Movement.
Hits	55% chance to block incoming damage.	Reflects 10% of incoming damage.
Power	Air	Darkness
Kills	Always archer + Impetus. (Can be boss)	Deals 25% of current tower hp as pure damage. (Gets multiplied with Era shield)
Hits	Immune to stun.	Invisible until close range.
Power	Light	Universal
Kills	Reduces resistance against all elements.	+100% damage split across all elements.
Hits	Shield that grants 1 sec. invulnerability.	40% chance to fully heal after lethal hit.