I like the idea of the gates, but they do seem too strong, and in some cases it can be WAY too easy to open them up. Not to mention that multiple monsters PER TURN can be..... bad.
I've a screenshot that shows an example of this going terribly wrong:
The tile that I'm standing on is the one that I actually purified; and then that gate, which is a good distance away, instantly opened, and farted out two power-4 monsters on one turn. This being AFTER I only just dealt with the starting monsters.... not only can I not possibly kill these, but I only have like 6 guys to begin with, two of which are scouts, and most of which are kinda wrecked. I havent the foggiest idea as to how I could survive that at all. Havent really built up much scrap to undo the destruction they might cause, either, and I cant really get at the gate itself without having to go through a forest of random monsters.
Fortunately I've been backing up the game REALLY REALLY OFTEN since I figured something like this was bound to happen (and starting over for the billionth time kinda interferes with testing stuff), but yeah, I'm thinking those are a bit.... off.... at the moment. This is on the default difficulty, for strategic.
EDIT: And I wanted to say, that I dont think the mechanic of caves and similar things auto-purifying should be changed here. That mechanic actually works out pretty well most of the time. Just the gates themselves need something done, though I dunno what that might be.... but I'm sure you guys will think of something good to do there, heh.
EDIT 2: Er.... wait. The text on these says it spawns ONE monster per turn.... is that what it's supposed to do? That would actually make a HUGE difference here.... it'd still be pretty freaking tough but I could see a way of possibly beating the thing if it werent spawning two of them at once.
Also, I do agree, these things are a really cool idea, and having threats come from places other than JUST Demonaica and his castle makes perfect sense within the game world. So these are good in concept and execution for the most part, just maybe a bit of balance issues.