It's not difficult to implement in any sense of the word, Aries.

It's as simple as this:


Player attempts to move into a bridge tile

If player on pallisade > step on top of the bridge
-If there's another player in bridge tile and on top of the bridge > halt movement(DENSITY ZOMG?!?!?)
If player on ground > step under bridge
-If there's another player in bridge tile and under the bridge > half movement(ZOMG AGAINA???! THAT WAS SO HARD!)

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Bridge becomes slightly transparent when someone steps under it. Only transparent enough for you to see that someone is under there. Then clicking bridge while someone is under it > target the person under it.

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