Land-bridges
Land Bridges in RTW
The only thing you need to adjust to make the land bridge work is map_heights.tga.
Map_ground_types.tga is only relevant in making sure you don't have forest or mountains in way that would block the path. Map_heights.hgt is not required in 1.5 or 1.6. Map_regions.tga does not control the coastline.
To adjust map_heights to make a land bridge you need to realise that only the pixels with completely odd numbered co-ordinates control what turns up as land or sea pixels in map_regions (map regions being half the size). To make a land bridge you need to get map_heights to generate a corrected_regions.tga which has the corners of two pixels from each island touching. The corrected_regions.tga is the 'real' map_regions that the game uses, trying to re-draw the coast on your map_regions won't change anything as heights controls the coastline entirely.
For example a land bridge from a modded map:
The two pixels marked in red are co-ordinates 33,195 and 35,195 they are the ones that make the land bridge because in the corrected_regions.tga they give:
the two diagonal corners touching.
You can create something that looks close enough to make land bridge in heights but if you don't have the two odd numbered co-ordinates it won't work.
The land pixels in map_heights do have to be 1,1,1 or lighter (not 0,0,0) but they can be lighter and the sea darker that doesn't effect the creation of the land-bridge only the appearance of the map, the presence of the other land/sea pixels on the even or odd/even pixels doesn't effect where units can walk only what coastline looks like.
Land Bridges in M2TW
In Medieval II: Total War the ability to add long land-bridges has been added. They are shown as green lines (RGB 0,255,0) on the map_features.tga.
The map_heights.tga method above can also be used however it is more difficult to tell in advance if the land-bridge will work as a corrected_regions.tga is not generated in M2TW. The old RTW method does have the advantage of not showing up as a bright green line on the campaign map.
The line of the land bridge should be only 1 pixel wide.
