squareevent
+class -2 "High Cultist"
+class -3 "High Cultist"
+chance 10
+terrain 22
newunits -3 "1*Spearman"
newunits -3 "1*Spearman"
endevent

This gives each High Cultist player in each castle they own a 10% chance for 2 spearman per turn.

You need to replace the number behind terrain with the respective number for coastal village (ctrl + i ) on the map and maybe make a second such event for deserted town or any other tile for spawning.

With Bakemono one could do the same thing for mines with respective number, but i think one cannot increase the spawn of their enchanted mines separately, as they have the same terrain number as unenchanted (but i just think that, maybe just check).

Any spontaneous spawning of units in certain terrain, in CoE mods, is indeed done through event modding. But... that’s one of the areas that was completely reworked in CoE4. (The other one is rituals.) So the solutions for CoE3 and CoE4 are incompatible. The one presented by Carn is for CoE4.
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CoE3 events are documented in the map editor manual, not the modding manual. Events can be used in maps as well as in mods (this is still true, in CoE4 too). In a map file (coem file), lone CoE3 event commands can be just thrown in anywhere. Whereas, in a CoE3 mod (c3m file), any sequence of event commands must be preceded by the command event and terminated with the command endevent. The actual CoE3 event command to use is classterrevent.

For examples of how this can be done, you might for instance take a look at the Rohan variant mod[coe3.proboards.com] I made, based on the Rohan mod by (TBS)Hunter.

For instance, the followingwill cause a “Rohan Princess” to appear in any town (terrain 17) owned by a player of the class “King of Rohan”, 1% of the turns.

Or, another example:will cause two spearmen, and a Captain commander, to appear – at once – in any iron mine (terrain 14) owned by someone of the Bakemono class, 1% of the turns. (You’ll need to add to add a classterrevent for each terrain type where you want this to happen, e.g. 15 for silver mine, 33 for gold stream, 34 for gold mine.)

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CoE4:

Originally posted by carn112004:

squareevent
+class -2 "High Cultist"
+class -3 "High Cultist"
+chance 10
+terrain 22
newunits -3 "1*Spearman"
newunits -3 "1*Spearman"
endevent

Triggered this way, the event will go off as many times per turn as there are High Cultists in the game, if I’m not mistaken. Which probably isn’t what you want. The trigger really needs to be replaced with a trigger. Except, of course, this won’t work with v4.16 due that command being currently bugged. Fix has been promised for the next patch. (In the meantime, it will work with a - instead of a +.)
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Edit: One more CoE3 example added.

Thanks Marlin, you are correct, my event only functions as intended if there is a single high cultist in game. Yours works with several.

thanks guys!

...And thanks to Johan, of course, who has now, in v4.17, fixed the +squareowner event trigger. 😀