Oh, and I just realized you wanted organization, too...so, here are a couple of attack/defense tactics that were fairly common.
On the attack, you'll want to have your archers at the front (so they have an unobstructed shot at the enemy), and your defensive infantry (spearmen) behind them, the two types forming a line to meet the enemy. On either flank, you'd have your 'shock troops': cavalry, heavy infantry, etc., to flank the enemy once they've been engaged by your defensive line. Long story short, you pepper them with arrows, send your spearmen forward, let them engage the enemy, and then flank with your shock troops.
To counter that, you'd want to make a long, long line with your non-archer infantry, and then concentrate your heaviest, toughest, bravest troops in your center. If you have any cavalry, keep them on either flank. Your archers would be behind your battle-line, and would try to shoot at the rear of the enemy formations once they've engaged your line. Then, you try to flank while your centry tries to push through the enemy spearmen.
Now, there are all sorts of counter/counter-counter tactics and strategies and formations, but the battle-line-with-cavalry-on-the-flanks was one of the more versatile. As well, the tactics with the flanks can be endlessly modified for specific situations (i.e. mass all the cavalry on one flank so they can charge down a particular hill), but they're too numerous to go into detail.