When developing the Visions of Zosimos Board Game, we wanted to make it as faithful to the PC game as possible. After prototyping the game on paper for so long it seemed so natural and simple. The one outlier though were the Ambushes. In engine, the Ambush card is placed on a tile and is visible to the player and invisible to the opponent. When paper protoyping, we relyed on the "honor system" and because we were just testing, it was totally fine. When pitching the idea to potential players, the honor system didn't cut it. Not only do people not trust their friends, but remembering where you placed your Ambush was another challenge which we designers faced as well.
We first tried placing "dummy" ambush tiles which were just two fake Ambushes and one real one. "Dummy" tiles that were stepped on would be removed from play changing the odds from 33% to 50% and so on. This was an interesting idea but also relyed on the honor system because a player could easily change the tile they used. We thought of having the real ambush have an identifier face down but was eventually scrapped. Then, instead of "dummy" tiles we placed three tiles that could be triggered and the player got to choose which one. The idea was that since the opponent knew where they were it would be fair to both players.. Sadly it was overpowered because cards with a large Trigger Radius could almost cover the entire board. The Ambush idea was almost scrapped entirely from the Board Game but we did come up with a reasonable solution.
In the Board Game version only, Ambush cards will be activated when a certain condition is met. So if an enemy moves two steps or makes a successful attack, you would flip your Ambush card that you played previously would activate. You don't have to worry about where the card was played and it forces you to pay more attention to your opponents actions. The issue with it is that you have to remember that the card was played and what the conditions were for the card but it is the best possible way to keep some really great cards in the game.