The seals are double-sided coins, a personal one you keep permanently and neutral seals you collect and spend throughout the game. Values on your personal seal are 2 on one side and 3 on the other, while the neutral seals are 0/5, 1/4, and 2/3. During the auction, the first player takes a seal of his choice and flips it (yes, flips it in the air like a coin) to determine his bid. The other player then contributes as many seals as he wants, flipping them in turn, until he passes the value of the first player. At that point, the first player can contribute more seals until he retakes the lead, going back and forth until there's a winner. Once you use your personal seal, you may contribute no more. All neutral seals used are discarded after the auction.
The auction is interesting because you can go for a big score and use the 0/5 seal if you have it, but you may flip the 0 side, while the personal seals offer a guaranteed 2 or 3 value, but you can then add no more. Of course, if you spend all your neutral seals, you're vulnerable in future auctions until you can acquire more, so you need to choose your battles carefully.
The action cards get you raw materials, allow you to process raw materials into finished goods, buy neutral seals, allow you to sell wool in order to get more coins, switch the start player, etc. Once a player has completed his actions, he may apply processed goods to his building at a cost of 2 coins for one, 6 for two and 12 to build an entire section of three. Additionally, the first player to build a particular section gets a bonus of coins, seals or a free resource conversion.
The game ends when a player has completed his building or when four passages of three rounds each are completed. The player who completed his building first, or whoever has the most complete building if neither is done, wins.
During the final turn, the game came down to one final card we both needed to win. The auction was close and would be determined by one final flip of the coin; whoever won the toss would be first to complete their building.
I won the toss, finished my fortress and won the game. It wasn't entirely satisfying winning like that, but it sure was exciting.
After four losses in a row, Bob wins Builders Duel.
Coming up: Mythological creatures race at the behest of a god in Odin's Ravens.