Jack_of_shadows
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If the bridge is impassable, the game should still prevent the player from simply sending their unit to the other side; it should be considered inaccessible. You would have to force the player to approach the bridge and somehow initiate the crossing (e.g., using a teleport function with no animation, so it instantly teleports – a common game convention). Alternatively, you could create a shallow river with a ford in that location, place a region there, and block it for the player, then unlock it after the bridge is built.