Yesterday I played my legacy save, with new heroes. I was playing lava mine, first on hard, then in insane with the same build. Hard was an easy win. The first wave of insane was on its way, and suddenly I had this message " your core is under attack". But there was no enemy around, and my core still had max health. Then I got the message again, and again and again and finally I lost. My defenses were still up, and the crystal still max health, not destroyed at all. WTF.
Replayed map. Same thing. Replayed map, Same thing. Filed bug report (invisible enemies destroying my core), went to bed.
Today before playing, I verified my game cache, then went to hard lava mines. No bug. Go back to insane..the same bug again.
So I gave up, and continued in hard.
Before stopping, I tried again, the same bug. Another try, and at last! I noticed a few goblins going in the wrong direction. I followed them and... There's a second core up there that I didn't notice!
Crap. I seem to remember something about this during the beta. Mystery solved.
But I do feel silly about the bug report