It is not that complicated to code, because it will use very similar ways to work.
The strategical map is a big map (I've tested it with up to 1000x1000 tiles), with all the possible terrains (at least in the epic game). City building, roads, mines, exploration, etc are done on this map, as in Civ. You can also move your units here, and make strategical bombardments. When you move an unit into an ennemy square, you can then plan a battle. You can decide to add several units, even from different squares, into the battle, included airplanes, ships or artillery.
Once every player has moved, come the battle resolution phase. If you decide to play the battle in details, a new map is opened. It will be much smaller (20x20 I think), the terrain will be the one from the strategical map, with some random variation.
The battle will start by placing your units. The available tiles will depends on the attacker position : if you attack from only S side, then the attacker will be able to place units in the 3 Southmost lines, and the defender in the 12 northmost lines. If attacked from S and N at the same time, attacker will get the 3 lines S and 3 North (of course, he can place only the units coming from this direction), and the defender will get the 6 lines in the middle, etc.
Then, you can move, attack, as in CivIII, except:
- There is no building / discovery / diplomacy during the battle, only move / attack / die.
- The range of units will be greatly increased. On the strategical map (for epic game, can be different for scenario), only a few units will have a range of 1, and very few a range of 2 (exemple : catapult, archer, riflemen = range 0, artillery, battleships = range 1). On a tactical map: archer : range 2, rifleman = range 4, catapult = range 4, artillery = range 10, battleship = unlimited...
- The units will be able to fire, or melee, with different stats, and different attack / defense against different ennemies.
Regarding the move, first the units will move faster on the tactical map than on the strategical, there is no supply issue on the tactical map (so no "don't go to far from your territory"), and the cost / bonus may be different.
Because on the strategical map, the units go relatively slowly (they need to get supply). So in desert, you can decide cavalry goes only 50% than infantry on the strategical map, but 150% faster in the tactical map, because there they can simply charge without resting horses, or spare water.