Great job here! The choice of symbiotic animals is cute and well executed. I like how the action system scales up nicely for a big endgame. I feel like it would make sense if some capability was lost when the Mound was destroyed (similar to the Duchy), but I'll defer the balance to the one who's play tested them ;-)
Like others have already said, the only thing I could see being an issue is matchups with limited buildings. If you were still concerned, you could look at using the Ruins as an optional target? I've always thought of the Ruins as something like a destroyed building, so it could work thematically (although it would be much less work to take it over). Of course, that pesky Vagabond (or Warlord) might make it a moot point, but more options could be better?