I know this topic has been discussed before but I haven't seen some of my thoughts mentioned. While being new, I have played both games for many hours. I started with RTW and played it for three weeks. I then got MTW and played that for five weeks and am now back to RTW for the last three weeks. I play about 35 hours a week. I know I have no life.
This is what I have noticed. I was able to win in RTW by using good military tactics like high ground, flanking attacks, missles to soften up enemy, etc. I think I lost only one battle when a green army went up against a slightly larger vetern army.
When I got MTW I lost my first four battles. It seemed like the AI was better but in fact it wasn't. The real difference is in unit handling, morale and other non-AI issues. RTW doesn't care if the archers are more then two ranks while MTW does. I can't see why it matters and IMHO RTW is more accurate in this. Also in MTW you needed long lines to wrap the enemies flank. It seems MTW units didn't turn to fight on the side. RTW is more accurate as units do this. Once I learned that a thin 2 man line is better then a 6 man deep line battles became easy. The other big difference is that units seemed to have their own mind in MTW. I tell a calvary unit to position itself on the flank but wait before charging. But even before getting to their position they chage all alone. After executing one or two generals that would have stopped in my armies. RTW doesn't nearly have as many units doing what they want when they want. MTW had calvary charging before the enemy was locked in front and their charge would not be on the back as they never went to my required position. All of this has nothing to do with AI but made the battle much harder. The only way to prevent this was to move the unit so far back that it took way to long to commit when they were needed.
Finally, I started with the Polish in MTW. They had poor generals and low morale, not to mention just average units. RTW starts with the Romans who have better generals, units and morale. Therefore the battles are easier. But once again this isn't the AI.
In both games I've seen the AI rush to get the high ground, try to attack my flanks, reposition their units to protect flanks, and also try to get beneficial unit to unit matchups. Neither was great but there is not much difference. The only place where there is a difference is in how the general is handled. In MTW the general is almost the last to partake in a battle. In RTW the AI general might be the first as it depends on the unit. And in RTW the loss of a general is a bigger morale loss and penality then in MTW. This is the only real difference and complaint about RTW over MTW that is justified.
The other big difference is in how the reinforcements are handled. MTW had a hard limit on units and the rest were reinforcements. This had the effect that you couldn't chase down a routed unit to destory it and make sure it never rallied. You might run into a fresh reinforcement and get caught off guard. This then led to units rallying and attacking again. In RTW you can keep chasing the unit preventing a rally and this then increases the chance of the whole army breaking. Also all units are on the map at the start and hence you can plan accordingly. I remember in MTW I was pressing a large army and winning. Since I was the attacker I had to move forward. However, their reinforcement point was slightly behind and on the flank. Poof, units appeared and had my flank. Not only doesn't this have anything to do with the AI it isn't even realistic.
Another big non-AI issue that makes it seem like MTW is harder then RTW is that in MTW the enemy can retreat and thats it. In RTW they can retreat but you can then pursue. If you have movement left the next attack is forced and the AI must fight you. This means many more of the battles are lopsided with my larger force getting to fight the smaller army since they had no choice to retreat a second time.
I'm playing RTW because the battles are really about the same once you learn the MTW secrets. And the strategic game is so much better in RTW. The interface makes managing 50 provinces so much easier then 20 cities in MTW. Not to mention the handling of agents and ships. I also like the strategic movement so much more in RTW. In MTW it was one province no matter the size.
I know this was long but I just wanted to throw my two cents out there.
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