[Edi]

In SP, combat happens at the end of the player's turn. Meaning if I play red and attack blue, that is resolved at the end of my turn. If he then attacks me, that plays at the end of his turn.

In network MP, I'm not quite sure how it happens, because if you're moving sequentially with your ally, you should be able to attack an enemy. Meaning if red and blue are allied, and red moves to attack yellow and then blue moves to attack the same stack, they attack together at the end of blue's turn but before yellow player moves.

Turn order is the color sequence. Red, blue, green, yellow, purple, teal, brown, white, which also corresponds to player numbers 0-7. It does not vary, but if you play with less than eight players, you can add all in game setup, then delete the colors you don't want from the in between slots.