bucketofsquid (US1) said:

To make a fire castellan you need to collect regular equipment.  Relic equipment is very powerful but does nothing to reduce fire damage.  Orange is the best, then purple and green does some but not much.  Look for % fire damage reduction in the benefit descriptions.  Some % values will be high and some will be low.  Some items will have multiple reduction benefits.  It is good to pair them with loot reduction benefits too.  Hitting towers from each of the 4 map zones is how I get mine.  Castellans can have the fire damage reduction too.

I don't remember the total cap on fire damage reduction but when I have one at or over the limit, it takes a massive, many wave attack to do damage to my buildings.

Just remember to move your troops out or you will take terrible losses.  You will also lose a lot of resources if there are any there.