The trouble is, even with relatively simple maths, it's easy for players to make mistakes and these mistakes become compounded in complex multi-move games. I could see a forum RTS working with simpler mechanics, with every player personally responsible for updating the map themselves.
So for instance, the map could start like this:
. .z
eP $$$$$
3$ $$$$$
e .'$$$
$$$$" e .
^* 4$$$$$$$$$$$
^*%$$$$$$$$$$
e$$$$$$$$$$
=dP$$$$$$$$"
*Fz$$$$$$$"
" $$$$$$$$.
"$$$$$$
**-""$$$$$$$c
d* 3$$$$$$$c
..$ " $$$$$$$$$F
$$$Ld$$ J$$$$$$$$$F
e$$$$$$$r $$$$FC$$$$$
J$$$$$$e. % 4$$$$$$r
$$..$$$$$$$$$ *$$$$$$$e
$$$$$$$$$$$$" P $$$$$$$$b
$$$$$$$$$$F 4$$$$$$$F
$$$$$$$$$$$$ $$$$$$$$$
^$$$$$$$$$$$ $$$$$$$$c
e$$$$$$$$$ $...4$$$$$$$$$b
$$$$$$$$$$ .$$$$$$$$$$$$$$
-""C$$$$$$$$ z"$$$$$$$$$$$$$" $b.
$$$$$$$$$$$% 4$$$$$$$$$$$$$$$$$F
C$$$$$$$$$$$ $$$$$$$$$$$$$$$$$$b
*$$$$$$$$ .d$$$$$$$$$$$$$$$$$$%
*)$$$ .$$$$$$$$$$$$$$$$$$$$"
^"" $"*$$$$$$$$$$$$$$$$^
"$".$$$$$$$$$P*
.. 4$$$$$$$$$$$$dF
.z$$$$$$$$$$$$$$$$P"
$$$$$$$$$$L**$*$""
d$$$P ""
.$*""*"
""
Then, red player, following some agreed rule, could expand his forces from Cornwall into Devon...
. .z
eP $$$$$
3$ $$$$$
e .'$$$
$$$$" e .
^* 4$$$$$$$$$$$
^*%$$$$$$$$$$
e$$$$$$$$$$
=dP$$$$$$$$"
*Fz$$$$$$$"
" $$$$$$$$.
"$$$$$$
**-""$$$$$$$c
d* 3$$$$$$$c
..$ " $$$$$$$$$F
$$$Ld$$ J$$$$$$$$$F
e$$$$$$$r $$$$FC$$$$$
J$$$$$$e. % 4$$$$$$r
$$..$$$$$$$$$ *$$$$$$$e
$$$$$$$$$$$$" P $$$$$$$$b
$$$$$$$$$$F 4$$$$$$$F
$$$$$$$$$$$$ $$$$$$$$$
^$$$$$$$$$$$ $$$$$$$$c
e$$$$$$$$$ $...4$$$$$$$$$b
$$$$$$$$$$ .$$$$$$$$$$$$$$
-""C$$$$$$$$ z"$$$$$$$$$$$$$" $b.
$$$$$$$$$$$% 4$$$$$$$$$$$$$$$$$F
C$$$$$$$$$$$ $$$$$$$$$$$$$$$$$$b
*$$$$$$$$ .d$$$$$$$$$$$$$$$$$$%
*)$$$ .$$$$$$$$$$$$$$$$$$$$"
^"" $"*$$$$$$$$$$$$$$$$^
"$".$$$$$$$$$P*
.. 4$$$$$$$$$$$$dF
.z$$$$$$$$$$$$$$$$P"
$$$$$$$$$$L**$*$""
d$$$P ""
.$*""*"
""
This way, the players would never have to wait for the GM to update anything and it could act in real time. The rules would have to be simple enough that players didn't often submit illegal moves.