Links between maps are basically teleports – you walk onto a tile that is a link, and you appear on a different map. In order to make a tile link to another map, add an "url" property to it, like this:
 tile = , url = village.map
From now on, the character "3" will behave just like the character "," in your map definitions, except that stepping on it will take you to the map "village.map". You can also link to a particular location on the other map:
 tile = , url = village.map#1
This will link to the first tile marked as "1" on that other map. Using digits for the teleports is not necessary, but is a nice convention.
You can even link to a map on a completely different wiki! Just provide a fill URL:
 tile = , url = http://example.com/village.map#1