Belief Takes Root

When you ¨Remove 3 Health worth of Invaders”, can you Remove 3 damaged Towns?
No, the Health of an Invader doesn't change when it takes Damage. Damaged Towns still count for 2 Health each.
A few effects (e.g., Kingdom of England Level 5, or some Event/Fear cards) modify Health, and this does affect how many Invaders you can Remove. But Damage does not reduce Health. (Health is simply the total amount of Damage it takes to destroy something.)
(Discussion elsewhere) Tags: Damage, Health, Seek Safety, Belief Takes Root, 14 more...

When an action removes N Health worth of Invaders scaling with a condition (e.g., the number of Dahan for Inspire the Release of Stolen Lands), do the Invaders have to be removed in batches of N Health?
No. Total up the Health limit on the removal and then remove a set of pieces that's at or under that limit.
This only applies if the Invaders are all being removed as part of a single action (e.g., in Inspire a Release of Stolen Lands or Wavering Resolve). If "each player" is removing Invaders (e.g., Seek Safety, Belief Takes Root, Depart the Dangerous Land), then that's one action per player – the totals can't be combined.
Tags: Health, Inspire the Release of Stolen Lands, Wavering Resolve, Seek Safety, 2 more...

Does Dahan on their Guard *(In each land, 1 Defense per Dahan)* provide Defense based on the number of Dahan at the time the Fear card was revealed or when the Ravage happens?
For island-wide effects like Dahan on their Guard, you check the number of Dahan there when the Invaders do damage (usually in the Ravage).
This applies to all Event / Fear cards that affect all lands with (some contents) - you treat it as a game-wide rule for the turn, so you don't have to keep track of where pieces used to be across the entire board. Powers which affect all lands of a particular type work the same way, for the same reason.
Powers which affect only one land check land contents at the moment of use, again to minimize required memory: it's easier to remember "this specific land has Defend 3" than "this land has Defend equal to the number of Dahan in it at the time".
Unusual cases:
- The Coordinated Strikes Event checks "are there 3 or more Dahan?" immediately to see where Damage happens, then again later on (when it's relevant) for the Defend.
- The Trees and Stones Speak of War Major Power provides per-Dahan Defense that moves with the Dahan if you hit the threshold (because the threshold explicitly says so).
  (Discussion elsewhere) (Discussion elsewhere) Tags: Defend, Dahan on their Guard, Belief Takes Root, Canny Defense, 11 more...