DISCUSSIONS on “Rules for online games in HoMM3 and formulas for calculating player rating.”

What if, during an attack, the defender goes first? For example, they cast an explosion spell, and the attacker can't retaliate or escape?

The restriction only applies to the attacking hero. I forgot to mention that. If the defender goes first, that's the attacker's problem. In the worst-case scenario, if they see that they'll be defeated in the second round without getting a turn, they can retreat without casting a spell.