Only common sac-terrain. I'll see how that feels and either make at least one more, or remove this to uncommon.

Theorycrafting, it seems like this is going to be a Seal of Rebuke that also says "You can't cast other (defensive) terrains". If attackers can choose to attack you through your Pre-battle Feastground rather than through this (so it effectively says "You can't cast any other terrains", that's quite a drawback in the set, while being no drawback at all in casual decks where this is the only terrain.

All of which is to say, I have a feeling this design has problems. But this is all tentative until I've played with it.

What you just said is right, but it seems like a problem with any good defensive terrain, not just this one.

I recognised that problem when I was trying to come up with the rules, and if it couldn't be fixed, I assumed terrains wouldn't work.

This is why I floated the idea that defending player chose which terrains to "block" with, since then you can always activate your strongest terrain only if you want. I thought that could be made intuitive with the right wording, and if not, I hoped to suggest something else similar. But if not, it may well torpedo the whole idea.

But do you think that's right, or do you think it's just the "ambush grounds" design that's problematic? (I've not played with them either, except on friday when defensive terrains didn't come up until too late.)

Is it possible to hook terrains together like lego pieces? For example, if my opponent played Feastground, could I say "Fine. On my turn, I cast Ambush Grounds and attach it to Feastground. When you attack through your Feastground, you always come out through my Ambush Grounds, and when I attack through my Ambush Grounds, I always come out through your Feastgrounds."

That way, you can make negative effect terrains, because you use them to make your opponent's positive effect terrains worse (if your opponent doesn't use terrains, all the better). It also cuts down on the total number of terrains you can attack through during a single combat, which you've already mentioned, can be problematic. The only downside I can see is one of timing: the first person to play a terrain could be setting themselves up. That might be fine, though, if the terrains are designed right.

Oh, and then there's the confusion issue in multiplayer games. Feastground might have three other terrains attached to it, one for each player. Go left from the Feastground, and you get to Billy's ambush ground. Go right, and you end up in Patty's Sewer Network. Confusing, yes. It also sounds fun, though, so maybe?

Yeah, I toyed with the idea of terrains slotting together and couldn't quite get it to work, but it would be very cool if it did. If the current rules still seem problematic, I want to revisit it.

That does sound rather cool :)

One simple fix to the issue is to give terrains subtypes for "Attacking" and "Defending", which lets you clarify the rules to be "When you attack, you may assign up to one attacking creature to each Attacking terrain you control; you must assign (at least?) one attacking creature to each Defending terrain defending player controls".

The playtest rules implicitly had two subtypes (I just didn't call them subtypes until I knew if that would be more or less confusing, it now seems I should have gone ahead and printed that, since most people seem to expect it). I think that's an absolute minimum necessary, since attacking in an opponent's Viashino Encampment is confusing whether or not it's ambiguous or broken.

I hoped to play a game with several terrains out at the same time and see whether the "multiple attacking/defending terrains" was a problem, and whether "multiple defensive terrains, one good, one weak" was a problem before fixing the rules further.

But for now, I'll assume two terrain subtypes, which hopefully removes the ambiguity, and wait and see if multiple defensive terrains feels right or not.

now it seems like a poor mans netrunner or guardians.

@amuseum You're probably right, but what do you mean? (I've not played netrunner yet, and don't even know whether guardians is a game or a card type...?)