Roundabout cardsWhen you use these cards, pay close attention to the order that you dealt out the Supply piles. They don't exist in a shapeless vacuum any more: they can look at the piles adjacent to them, or even further away! They all have the type "Roundabout", but that's just so you don't forget to pay attention to the words "to the left" and "to the right". There's no rules meaning to it. And they all share a common theme.
Pearl • $4 • Treasure - Roundabout
+$2
Cards from the three piles to the left of this cost $1 less this turn.
Circlet • $2 • Treasure - Roundabout
Choose one: +$2 , or gain a copy of the card to the left of this.
-
When you buy this, you may discard a card of the pile to the left of this. If you don't, trash this.
Smuggling Ring • $5 • Victory - Roundabout
Worth 1VP per 2 cards of the pile to the left of this in your deck.
Round Table • $6 • Action - Command - Roundabout
Play the next non-Command Action to the left of this costing up to $3, leaving it in the Supply.
Play the next non-Command Action to the right of this costing up to $3, leaving it in the Supply.
Calliope • $3 • Action - Roundabout
Trash a card from your hand. Gain the next card to its left that costs $0 to $2 more than it.
Waterwheel • $3 • Project - Roundabout
After you play an Action card, if the pile to its left is an Action, you may play a card of that pile from your hand.
Notional FAQ:During Setup, deal out the piles from left to right. Every Kingdom card has one card to its left and one to its right, because the order is a circle; even if you've arranged your cards in a line, consider the rightmost card to be one step left of the leftmost card. (That's why they're Roundabout cards, and not Straightforward cards.)
Only Kingdom cards have an ordering; Base cards, non-Supply cards, Events, Projects, tokens, and any other things that have wandered onto the gaming table do not have anything to their left or right.
"The pile to the left of this" means that Supply pile, even if it's empty. "A card of a pile" means a card that started the game in that pile.
"The next non-Command Action to the left of this" means keep looking to the left until you find a card on top of its Supply pile that qualifies. This may refer to a different card over the course of the game, as the contents of piles change. It may loop around and refer to the pile itself, if no other card qualifies, and failing that it'll refer to nothing at all.
Design Notes:These cards aren't really balanced. But they show off what's possible with this mechanic.
Sorry about the grammar in "a card of that pile". "From" is more correct, but unfortunately "play a card from a pile" already means something in Dominion.
The design space of Dominion is constrained by the fact that any particular card can only rely on having itself and the Base Cards available. Duke can count the number of Duchies you have - but you can't make a card like it that counts Villages, because what happens in a game without Villages? Roundabouts provide a loophole: put Smuggling Ring to the right of Village, and you're done!
The card in Dominion that almost uses this mechanic is Young Witch. Although she couldn't quite be errata'd to a Roundabout card as I've written them here, since her behaviour around split piles and empty piles is different.
Smuggling Ring's name definitely 100% fits the "circle" theme. Rings are circles.