Feature or a bug: Serpents are completely invisible if parked underneath a vertical tower bridge.
Of course, there is still a colored dot on the minimap that marks the enemy serpent (or stack actually, the serpent just has to be the top unit to hide the entire stack), but this is easy to miss.
I don't think it is a good idea to require a player to check all bridges on the minimap in order to avoid an ambush by serpents.
The problem: It is just another thing that slows me down in making my turns. The game should not be a boring exercise in diligence. Either invisibility becomes feature, in which case the revealing dot on the minimap and the ability to click on the enemy invisible serpent should be removed, or at least the flag of the stack should stick out of the bridge somewhere.
Another possibility would be to have bridges become like the income site buildings that display their own flag of the nation that last moved through them - just like watch towers do right now.