Cheat?

I’m a newish player, just easing into multiplayer battles, and have been pounded a few times now. Watching the replay, there were a couple of attacks where the opponent’s troops just magically appeared inside my walls, as if transported down from the Enterprise. Makes me feel kinda dumb for even bothering with walls.

Is this a cheat? A feature I don’t know about? Sorry if I sound hopelessly naive, but I generally stick to single-player campaigns in other games because I’m not interested in cheating or cheaters. Thought BHG might be a better-tended cesspool, but….