1) It's going to be played a bit like a regular D&D game, and I assume also like most PBP games here. You control the battlelord... that's you. You give orders, your orders are carried out. You say March, they march. You say Halt, they halt. You find the enemy, and you say kill... then the fight happens. Preferably, when you say kill, you also detail some tactics as well.
Finding the enemy might be tricky, of course... and ambushes are deadly. Scouting will be important. Divination magic might be helpful.
2) There will be a map. I'm still open to suggestions for features you want on it. The terrain in the two countries will NOT be the same, and the terrain between the two countries will be rough going... I'm thinking there'll be an ocean off to one side, a river off to another, and between them will be a desert and a mountain range.
In this I'm borrowing a little from C.S. Lewis's "A Horse and His Boy"... Calormen wanted to invade Archenland and Narnia, but there was a desert in the way. Lack of water makes having an army cross it difficult... there's an oasis in the middle, but that can't support large numbers of men and horses. So the best way was to go off at an angle to reach a river, which would supply the water needed to get across.
The mountain range I would throw in because it adds yet another alternative to the desert crossing problem, and of course history has all kinds of cool war stories involving crossing armies over mountains.
So I've got the MIDDLE terrain pretty sorted out in my head. I just need some cool ideas for terrain in the actual countries themselves... cities on hills are cool, as are cities set on river islands. Interesting plataues? Canyons? Forests? Swamps? Moors? What say you?
3) "Large Towns" and larger have castles and fortresses associated with them, according to the DMG. I'll let you pick from those for a home castle. No need to buy one. Of course, you may not need much use of your castle either, depending on how much on the offensive you go. In "How to Defend a Mountain Fortress" 90% of the suggestions included the phrase "Don't play defensive. Get out of your castle and be proactive about this! Sitting still will get you dead!"
4) All good players are automatically aligned, as are all evil players. This is why good players get their own thread seperate from evil players... because you'll be communicating with your teammates, but not with the enemy.
If you so desire, you can coordinate attacks and things. If you do not so desire, and want more glory for yourself, you could go off and do your own thing. This is D&D, but at the same time, slightly different. It keeps the "do what you want" style of play, but has optional instead of mandated cooperation. Of course... cooperation on your team will likely help considerably...
On the other hand, there are to be no alliances with the enemy. You both want each other dead.