Ok, I think this is an AWESOME feature and something that is needed in this game badly! Most people are thinking of a drawbridge as an accessory to a castle, and that isnt the right way to look at this…
Having something you can walk over, and then switching it to NOT allow walking over it is the key. Let me explain. If you start a game on a small island, nothing touching land, everything you need on the island to survive. play far into the game, nothing ever touching land
the game quickly ‘breaks’ itself. most invasions cant even reach you so they just kinda hang out at their base ( and reproduce CRAZY by the way, like unbeatable crazy lol ) . if you touch to the mainland ‘late’ into the game. They finally find a way to get to you and swarm you crazy lol. Once you touch the ‘mainland’ the computer knows how to get to your area. Further invasions happen around that point, there is no ‘cut-off’ besides sending workers to MINE the connection to mainland.
I use the Island to explain because basically the computer thinks of your town that way. If you start on a ‘shelf’ in the mountains. Again basically your on an island of land. Larger chunk of land, some of your ‘shelf’ streches around the mountain… even being far enough from your town to have a goblin camp spawn on it or some other monster. If you never build a ladder/stairs up or down a ‘shelf of land’ … you would never have to deal with anything on another ‘level/shelf’ of land. Monsters dont build ladders 
if you start in the middle of a flat field, and completly surround your town with wooden picket fence, no gates. Again, your on an island. Might as well be surrounded in water
You dont go past your fence, and monsters will never come to break it. That all changes when you throw up a gate or door on this fence, mobs will come to break that. And will swarm your town.
So getting back to Drawbridges, I hope you understand now why something that is ‘toggle-able’ being able to walk across or not, would be a great addition to this game. If the AI of the game knows how to get to your town ( like thinking it needs to bust 3 gates and its there ) the game plays normal. If the computer AI DOESNT know how to get to your town (ie. you built walls, or fences no gate, or water blockage. ) it seems to trigger it into wired monster growth patterns that get crazy and unbeatable.
That is just my 2 cents worth. I hope we eventually see a drawbridge that lets me…
. build the castle of my dreams lol
. lets the AI know the path mechanics of a drawbridge, it isnt down yet cough AMBUSH cough lol
. gives me the ‘defense’ piece I have needed! Ill let the mobs in slowly, defeat them bit by bit! New Legendary difficulty? HA! I laugh at you now 