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Last Asylum: Plague Rat Swarm Strategy Guide (How to Beat the Rat King)
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For Last Asylum: Plague Doctors who want to know exactly which towers to upgrade, how to position heroes, and when to attempt the Rat King so investment converts into actual clears instead of failed attempts.
Rat Swarm is the progression gate everything else feeds into...
The Rat Swarm is Last Asylum: Plague's core PvE tower defense mode. Waves of infected rats advance along fixed paths toward your Sanctuary. You defend with towers built at key positions on the map and up to 5 heroes deployed on the field.
The Rat King boss at the end of each difficulty tier is the hard progression gate. Clearing it unlocks better rewards, higher difficulty tiers, and in some cases event scoring eligibility. Failing it wastes tower repair materials and hero stamina.
For Doctors investing $1,000 or more per month, getting stuck on the Rat King is not a strategy problem. It is an investment sequencing problem: hero concentration is too spread, tower positioning is not leveraging choke points, or active abilities are being used on trash waves instead of saved for the boss.
Tower placement (concentration beats coverage)
The most consistent Rat Swarm mistake at every investment level is distributing tower upgrades across the full map. A Level 5 tower at a choke point where two or three enemy paths converge produces more effective defense than five Level 2 towers spread along the entire route.
Identify the two or three natural choke points on your Rat Swarm map where paths narrow or converge. Upgrade those positions fully before touching towers in open areas. The rats that reach your Sanctuary through an undefended side route are almost always coming through a position that would have been covered by the same Diamonds spent on a weak tower somewhere else.
Tower type priority: damage towers at choke points first, slow-effect towers second at the bottleneck before your primary kill zone, area-effect towers third for clusters that break through the main choke. Do not invert this. A slow tower without a damage tower behind it delays rats without killing them. The combination in the right order creates a kill zone.
Best hero lineup for Last Asylum Rat Swarm
Rat Swarm rewards sustained survivability combined with burst damage timed for boss waves. The Rat King has a large health pool and hits hard. Your formation needs to endure contact long enough for damage output to accumulate. Teams that max damage at the expense of survivability fail on the Rat King wave even when they clear pre-boss waves cleanly.
Slot 1 (Frontline): Arthur (UR)
Arthur's Earthshattering passive reduces physical damage taken by the entire team. His frontline presence is what keeps Marlena in the backline alive long enough to output sustained damage. At max star levels, Arthur's protection allows formations to survive Rat King contact windows that would wipe lower-durability frontlines. Invest in Arthur first before any other hero in this mode.
Slot 2 (Secondary Frontline): Bella or Griffith
A secondary tank in the second slot extends the formation's contact window before boss waves reach the backline. Griffith provides additional durability. Bella is viable if Griffith is not yet built. Either hero in this slot reduces the number of failed Rat King attempts caused by Arthur being overwhelmed during the boss phase.
Slot 3 (Primary Damage): Marlena (UR)
Marlena stays in the middle-to-back of the formation, outputting sustained damage while the frontline absorbs contact. Her active ability should be held for the Rat King wave, not used on trash clusters where auto-attacks are sufficient. One manual Marlena ability activation timed to the Rat King's first appearance produces more total damage than two activations split across earlier waves.
Slot 4 (Support): Stellar (SSR)
Stellar's support buffs extend the formation's effective combat window in Rat Swarm specifically. Her healing or buff ability reduces the resource drain of repeated attempts by keeping heroes alive longer into boss waves. Stellar is the priority SSR investment for this mode.
Slot 5 (Flex Damage): Claire, Bestar, or Grenwald
Use whichever of these three you have at the highest current star level. Claire and Bestar both provide reliable secondary damage. Grenwald adds burst. The fifth slot adapts to your roster's current state.
Positioning rule: tank in front, support in the middle, damage dealers in the back. Putting Marlena in a frontline slot exposes her to early wave contact before she can output damage. One wrong slot assignment can prevent a clear that the same heroes in the correct positions would achieve.
How to beat the Rat King
Three rules apply specifically to the boss wave.
Hold all hero active abilities through pre-boss waves. Let your formation auto-attack through waves before the Rat King. Every ability used on a trash cluster is an ability on cooldown when the boss spawns. Your burst window on the Rat King is the first 10 to 15 seconds when his health is fresh and your formation is intact. Arthur and Marlena's active abilities should both land in that window.
Do not attempt the Rat King until pre-boss waves clear comfortably. If your formation takes significant damage on waves 3 to 5 before the boss appears, it will not survive the Rat King. Failed attempts waste tower repair materials and hero stamina that compound across repeated attempts. Invest more in your core heroes first. The correct test: if pre-boss waves clear with most of your team's health above 50%, attempt the Rat King. If not, invest more first.
Repair all towers between failed attempts. Towers damaged during a Rat King failure perform below their level output in the next attempt if not repaired. The repair cost is lower than the cost of another underpowered run.
Investment priority for Rat Swarm progression
For accounts investing $500 or more per month, Rat Swarm difficulty clears are determined almost entirely by hero star rank, in that order above any other variable. Tower positioning matters. Formation placement matters. But a higher-star Arthur and Marlena clear difficulty tiers that the same heroes at lower star ranks cannot, regardless of how optimally the towers are placed.
The investment sequence that produces the fastest Rat Swarm tier progression: Arthur to competitive star level first, Marlena second, Stellar third, secondary frontline fourth, flex fifth. Broad roster investment across heroes who are not in these five slots does not improve Rat Swarm performance. Five heroes at 2-star rank clear worse than three heroes at 4-star with two undeveloped fillers.
Last Asylum alliance coordination for Rat Swarm
The best alliance leaders do not just clear the Rat King themselves. They get the whole roster clearing it faster.
Share tower layout screenshots in alliance chat. A screenshot of a working choke point configuration is more useful than any written description. Members who copy a proven layout skip weeks of trial and error.
Publish a standard Rat Swarm hero priority list. When your alliance agrees on Arthur first and Marlena second, members stop wasting resources building off-meta heroes for this mode. One shared priority list prevents thirty members from making thirty separate investment decisions that work against each other.
Track which members are stuck at which Rat King difficulty tier. Members who are blocked on a specific tier almost always have the same problem: hero investment spread too wide, not concentrated enough on the core three. Targeted coaching on concentration beats generic advice.
Time Rat Swarm pushes around event windows where Rat Swarm performance generates event scoring. The same clearing effort produces additional rewards when aligned with the correct event phase.
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Last Asylum: Plague Rat Swarm FAQs
What is the Rat Swarm in Last Asylum: Plague?
Rat Swarm is the core PvE tower defense mode where waves of infected rats attack your Sanctuary along fixed paths. You defend with heroes positioned on the field and towers built at choke points along the route. Each difficulty tier ends with the Rat King boss wave. Clearing the Rat King unlocks better rewards and higher difficulty tiers.
What is the best team for Last Asylum: Plague Rat Swarm?
Arthur (UR tank, frontline slot 1), a secondary tank like Griffith or Bella (slot 2), Marlena (UR damage, slot 3 to 4), Stellar (SSR support, middle), and Claire, Bestar, or Grenwald as flex fifth. Tank in front, support in the middle, damage in the back. Arthur's frontline protection keeps Marlena alive long enough to output damage against the Rat King.
How do I beat the Rat King in Last Asylum: Plague?
Save all hero active abilities through pre-boss trash waves. Use Arthur and Marlena's actives in the first 10 to 15 seconds of the Rat King wave. Do not attempt the Rat King until pre-boss waves clear with most hero health above 50%. Repair damaged towers between attempts.
Which towers should I upgrade first in Last Asylum: Plague Rat Swarm?
Identify the two to three choke points where enemy paths converge and upgrade those towers fully before touching open-area positions. Damage towers at choke points first, slow-effect towers second at the bottleneck before the kill zone, area-effect towers third. Concentration beats coverage at every investment level.
Why can't I beat the Rat King in Last Asylum: Plague?
The most common causes: active abilities used on trash waves instead of saved for the boss, hero investment too spread across many heroes instead of concentrated on Arthur and Marlena, formation positioning wrong (damage dealers in frontline slots absorbing contact before they fire), or towers not concentrated at choke points. Address these in order before attempting the next tier.
What heroes should I invest in for Rat Swarm in Last Asylum: Plague?
Arthur first, Marlena second, Stellar third. These three determine whether Rat King clears happen. The UR heroes guide covers investment priority across all four UR heroes in detail. Do not spread investment across the full roster before these three are at competitive star levels.
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