Current principles for Terrains:

1. Still in early design, not worried about the rules not working, want to see if the most basic card ideas function as "obvious" and "fun" or not.
2. Assume terrains have a mana cost, and not a land drop, and a radically different card frame (like planeswalkers).
3. Assume your terrains stay in front of you (whether there's a notion of "attached to" or not).
4. Attacking creatures may attack through "good" terrain controlled by the attacking player and must attack through "bad" terrains controlled by defending player. One attacking creature per terrain. Blocking creatures are in the same terrain as the creature they block.
5. Currently have two common terrains per colour so we can test them thoroughly. Expect to drop down to one per colour later
6. Will adjust the rules later when we see what works and what doesn't. Especially want to check:
   • are they fun?
   • are they clear?
   • are they overpowered/underpowered?
   • are both attacking terrains and defending terrains interesting?
   • do they work at common?
   • how many are too many?
   • is it too annoying if there's no built-in answer to terrains, or is "playing more creatures" enough?
   • is it obvious which terrains you can/must use (even if the rules aren't specified yet?)
   • is there a simple implementation of the rules
   • is it balanced and intuitive that terrains affect flying creatures?

See also discussion in [Conversations] and [Multiverse Design Challenge]

Comments and questions on Comments on Terrains

I tried to make this a details page but couldn't create one for some reason. Alex, do you know what's wrong?

Brief playtest. Nothing stands out as especially good or bad, but sandwurms and terrains came up only briefly.

As expected beneficial terrains were very strong, like equipment without an equip cost. That's fine, I want to know if they're fun (answer: not sure), they can easily be toned down. Defensive terrains didn't come up.

Details page creation ought to work now.

could you label them so that rather than just being a terrain they would have subtypes ? i.e.
Terrain - Defensive & Terrain - Offensive (or Assault or something like it) ?
Just so it is clear to players?

If it helps, then definitely, but I'm not sure if it's necessary or not -- eg. if terrains say "a creature you control attacking in ~ gets +2/+0" or "a creature attacking you in ~ gets -2/-0" they will presumably get the idea and expect the former to affect their attack and the other to affect their defence. I want to try it and see what feels intuitive, and then try to make the rules match that, whether that's officially having two sorts of terrain, or spelling out "a creature must/may attack in this", or allowing players to specify which sorts of terrain are "active" (like choosing which creatures to block with).