Chokepoint and Province Access Map

[TinCow]

In my experience, the AI only does naval invasions if they do not share a land border with their enemy. The farther away the enemy's lands are, the more likely a naval invasion is to occur. They do happen though, and I've had to defend against them in all of my Legendary games (bar my Oda failures). They almost always land forces behind my front line, often far behind near completely undefended castles. Thus, naval invasions mean that war with a distant clan is still a threat that needs to be taken into account when making strategic moves. Due to the inability to reload from an earlier save, this in particular makes navies more important on Legendary difficulty. If you have a castle that is vulnerable to a naval invasion and cannot get an army there to defend it in 1 to 2 turns, you need to give serious consideration to keeping a fleet in whatever position is necessary to intercept any naval invasion. For example, when I play Date, I typically keep one fleet on each of the two main coastlines. The fleet holds position off the coastline of my front line province and advances with the front line. The job of those fleets is to keep any naval invasions from slipping behind the front line to land in my rear provinces.

You could take away all the lines off the map and use a "clean" map and place markers on that one for ambushes and bridges.

Without the routes, though, would that information even be useful? There are many bridges and ambush sites that are totally useless because the AI never goes anywhere near them. The key would be to list only the ones that are worth giving some strategic consideration to, which to me requires being able to see where they are places with respect to the access routes.