Trade routes sometimes leave limitations to build a good defense. I propose that a wall be built over the trade route; It would be good for treated mode rather than standard games.
I made this proposal in the following forum and received good support::
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Sure, why not?
I donât really have a problem with trade routes since even tho I canât wall on top of them I can wall all around them, so not bothering for me, but it would be nice anyway.
If you remember in the Chayton campaign there is a trade route coming out of a walled base (First mission, I think it came out of a gate, maybe just walls around them however).
By the way, you messed up a part of the poll, so we canât vote yet hehehâŠ
Walls are strong enough already, so no.
Thanks. Fixed up. First time I use this tool.
In some campaign missions it can be seen. Why not make it viable in multiplayer?Age of Empires 3 - 18 - Hold the Fort Walkthrough PC - YouTube
Cause it would be ridiculously easy to defend TPs.
Understandable in supremacy. I understand your point, but in late game I mean, not at the beginning of the game where it would be very hard. In addition, native trading posts can be locked up and also count as a way to win by monopoly. There are maps where there are more abundance of native towns than commercial posts on the route.
Self-opinion to revive this issue. 
I thought the wall over the trade route is not very relevant in the late game.
I donât like the Daimyo going to the rear with 5 mortars to destroy the factories in 1 minute after finishing the treaty.
No. If I wanted to play a slow wall-fest Iâd play AOE2.
Thatâs why I say late game.
I think itâs difficult to implement technically. You cannot create independent doors in the game. First a wall and then a door. This is how walls work in Age of Empires 3.
It is a simple change in the code. I have played mods that do.
Does this mod add a new âgateâ building or just allow walls to be placed on trade routes ?
In the first case, there is no more space for a new building in the interface of certain european villagers (Dutch).
In the second case, it would be possible to build a wall without a door on a trade route. I think that itâs a bit dangerous for trains xD
In fact, the solution could be: only the âgreat wallâ can be placed on trade routes and that they automatically transform into a door at this time (with an additional wood cost) 
Maybe a technology or a card can allow that ? It will be necessary to indicate somewhere to the player the functioning of this particular system (in the description of the technology) and to avoid a too easy defense of trading posts at the beginning of the game (an age 4 technology ?)
It auto designates when hovering over the path when you are going to build a wall over the path. I donât see a need for letters for something so simple.
I feel your pain here, although I think your real issue would be with balancing Japanese daymyos and not with the walls themselves. Then again, Japanese are their own can of wormsâŠ
Yes but in exchange increase wall cost and remove pillarless walls. Gl getting the latter to happen though. Mah hidden skills tricks1111!!!
I like that the walls have limitations so that you do not have control of the trading posts easily, but in the late game a good defense is necessary because there are skirmishes everywhere. On the map where this problem most manifests itself is on the âGreat Lakesâ map. It is difficult to make a good defense on that map because the trade route is a public highway.
This ability could be unlocked in supremacy by declaring a commercial monopoly.
How about this as a compromise:
Wall foundations can always be put across trade routes (for the cost of a wall + gate) and are immediately gates upon construction. However, trade routes function like cracked ground from AoE2 causing buildings constructed on them to take extra damage. That would make it still a weak point but not as bad as it is now.
On the topic of walls, pillarless walls need to be nixed. One way to do that would be to make pillars have slightly more health than walls, and any walls that connect to a pillar are bumped up to that higher health.
What I would do is extend the commercial monopoly to 10 minutes as it was before. So the other player will have more time to act.