I had a quick few seconds to pull up this information:
Help level: 113
Syntax: mapedit sector
Syntax: mapedit save
The mapedit command is used to make changes to the overland map system.
It's a fairly involved portion of the OLC code as a whole, but is not
terribly difficult to use.
Typing 'mapedit' with no other arguments toggles your editor on or off.
When on, any movement you make will change the sector you're moving to
into whatever you have yourself set for. Think of it as "painting".
To set yourself to create a sector, simply type 'mapedit sector '
where the type is something from the sectortypes list. Also note that
while you are in edit mode, typing 'look' will cause the sector you are
in to be changed.
To save changes to a map, type 'mapedit save '. If you leave out
the mapname, the map you are currently on will be the one saved.
For details on using the floodfill option, see 'help floodfill'.
For details on manipulating exits on the map, see 'help setexit'.
For details on manipulating landmarks on the map, see 'help setmark'.
See also: SECTORTYPES, REDIT, FLOODFILL, SETMARK, SETEXIT
You can get help on those other topics, but this next one is what you should look at as far as linking areas go:
Help level: 106
Syntax: setexit create
Syntax: setexit delete
Syntax: setexit map
The setexit command allows you to manipulate exits from the overland
to rooms or other maps. The command is fairly straightforward.
To create a new exit, simply type 'setexit create' while standing on
your overland map.
To delete an existing exit, simply type 'setexit delete' while standing
on the coordinates of the exit sector. You will probably need to use the
'coords' command to move onto the exit sector.
To set an exit from the map into a regular zone, type
'setexit ' where vnum is the target room's vnum.
To set an exit from one map to another map, type
'setexit map ' where mapname is the name you gave to
the target map, x is the target x coordinate, and y is the target y
Any manipulation of an exit automatically saves the entrances.dat file
to disk, so there is no need to worry about saving that file.
See also: COORDS, MAPEDIT
Also for the heck of it:
Help level: 103
In order to make a watch tower, the room leading to it must have an
exit leading up that is flagged both Overland and NoMob. The exit
also needs coords for the map to display.
See Also: ROOMFLAGS, REDIT
I think the watchtowers one was included in HELP ROOMFLAGS, but I moved it to its own help file to clean up a bit.
This should give you enough information to go on.