Turn order in battle.

[/U]Let's take a specific example. Suppose two heroes are fighting. The attacker has Champions and Royal Griffins in the first and fifth stacks, while the defender has Death Knights and Vampire Lords in the third and fourth stacks. All of these units have the same speed - 9. Who goes first? Does the number of units in all seven stacks matter?

In the second game, the number of units in the other stacks definitely matters.
For example, if the defender has Phoenixes and the attacker has Titans, the turn order will be Phoenixes-Titans-Death Knights-Champions, etc. (if there were no Phoenixes and Titans, Champions would go before Death Knights). In other words, the "snake" rule applies - the last turn was the attacker's, so with equal speed, the next turn is the defender's.