Why can not I defend city upon enemy attack?

Pls, why RTW let the enemy siege city first, why cant we fight back upon attack? thanks

Actually you can do it.Just attack any enemy army close to your cities in your turn.

You can.... Scouting and catching them befor they are at your walls. A Spy with some skills added to an army or city will expand its range of sight, in some cases by alot.

but personally i do prefer defend to attack however!

Well, in order to defend they have to attack. Laying a siege on a city doesn't automatically mean that they are attacking. First they'll build some siege equpiment and then starve you to death. If you're lucky, they'll attack. If they don't you can always sally forth (a rather good choice, if the AI doesn't decides to intercept your attack by rushing your forces to the walls and kill them before they even can fight. But then again, you can always put archers on the walls. That way they will die a horrible and painful death *mwhahaha*) or you can attack them with another army outside the city (this is probably the prefered choice for me, but you risk ending up with AI Controlled Reinforcements, which basically means that the AI general will make a charge for glory and honour against a wall of spears.)

i see, thanks. so that is the game rule. I have thought i made mistake somewhere. Another question: can i train or retrain during the siege?

Unfortunately you cannot train, retrain or construct during a seige.