Here is a list of game fixes you can try if you are unable to connect, at the bottom is a list of ports that should be open. I got this list from Stig and others who have tried to help others connect to the MP lobby.

"Failed to Login"

This means that your computer simply can't connect to the GS lobby. It can have more then one reason, but most of the time it has something to do with your firewall.
To solve it check this:
-Do you have a fast connection? Broadband (DSL or Cable) and over 128 kb/sec. It's recommended to have this connection or better, since slower connections slow down the game, not just for you, but also for the other players. If you still have Dail-Up it's not recommended to play MP at all, since you will terribly lag. 1v1 battles with smallest unit sizes might still work tho, but it's better to avoid it.
-Do you still have the firewall one?
-Do you still have background programs running?
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"Can't connect to host" (or similar)

This used to happen often in RTW and I've had it once in MTW2 so far. It has to do with the host having a bad connection. He can have Alt-tabbed out of the game, he can have background programs running or simply have spyware running. This means you can't join the game. If that really is the game you want to be joining, try again sometime later. Maybe he's back in game, or has simply clicked on the screen, which will bring him back into the game instead of the background program (msn, whatever).
If people tell you this often happens when they want to join you, but you don't Alt-tab out, do this:
-Don't leave MSN (or similar) on when going in game. It causes problems. When someone starts talking to you you get a small pop-up in the lower right corner, this alone is enough to disturb the game. Also close any other background program. I myself always have Startmonitor running, but this too can cause problems, if a new background program wants to start running. Most of the times you then will have a high change of being infected by Spyware, this can be normal and doesn't always have to do with pr0nsites or similar. If you want any advice on good anti-spyware programs, download Hijackthis.
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"Failed to join game" (or similar)

This is also a quite common problem. It has to do with having a bad connection at some points. With wireless DSL you don't always have the same good connection, so sometimes you might have problems joining up.
Best thing to do is, log out and log in again, most of the times it works. If you then can't log in again, you know it's a connection problem, simply close the game, start up internet, make connection and start the game again.
Personally I have had these problems only after I Alt-tabbed out. And that isn't recommended to do.
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"Game stalled, network problems for player ..." (or similar)

This means someone has network problems (obviously). This can be caused by having background programs running, having a bad connection or Alt-tabbing out of the game (yeah some do that in battle). Just wait a while till he gets back and the game will be resumed. Sometimes it's only caused by MSN in which case one click in screen is enough to get back into the game (so if the player with a problem wants to send a command), sometimes he's taken to desktop in which case it will take some more time.
It can also simply be so that his connection failed. In which case wait 30 secs to 1 minute. If he isn't back by then hope the host kicks him, if the host doesn't do that, I personally leave the game. If you however are willing to wait longer, no-one is stopping you.
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"NAT communication error" (or similar)

This will most likely only happen if you're connected to a network of computer and share the internet with them (as afaik NAT has to do with that). One of those persons is me, and therefore I'm getting this error from time to time. I share the same internet with loads of people, but we all have different IP addresses, and that's what's causing it afaik. For me to easy problem to solve it is logging out and logging in again. I've had this problem trice now, and all the times that worked.
At the moment I don't know what's causing it (apart from having a network inside a network, or something like that), but I do have the easy fix already.


From Sega:

Due to some compatibility issues with different versions of the game, some users may experience difficulties playing multiplayer games. Specifically, users with download versions of the game will temporarily not be able to play against users with DVD versions (and vice versa). Users with different language versions may also experience trouble.

SEGA and The Creative Assembly are working on this issue and will post an update as soon as it has been resolved. Until then, please know you should have no problems playing the single player campaign
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"Can't see games in the lobby"

Right you are having this problem. Till know I heard of 2 versions of the story

1. You bought the game at direct2drive.com or Steam or similar and downloaded from there.
2. Your DVD isn't working so you decided to get a tool so you can play without the DVD, but still installing it in a legal fashion.

For 1:
Best now still is to contact direct2drive and see what they can do. I don't know how it works, downloading, installing playing without DVD, but my guess is that it's linked to #2. Therefore your problem might be solved with the same fix for #2. However there is a difference between the problems, so it's best to ask the direct2drive or Steam if they already know the fix, they might have had it with other games too. Since the problem has something to do with the GS lobby.

For2:
Well you can play the game now, but MP is out of order. You can simply buy a new DVD-rom drive or wait till one of the Tech Mods comes with a fix. Best now is to wait. Maybe the next patch might miracly solve this, but personally I doubt that, wait till there is a solution, till then try finishing the Campaign.
It might have to do with your type of drive, as someone on TWcenter simply bought a new drive and now all seems to work for him.



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"Failed to login - Invalid Player Profile"

Remember that you need to enter the following:
your account name, the corresponding emailaddres, the corresponding password.
First check if you all did this correctly (some people also typed the cd-key in the Password line). Make sure you didn't make typing errors, as they can make sure it isn't working.
If it still doesn't work, try this:
-go to www.gamespyid.com
-login using your email and password
-click the username
-if you haven't verified your account do so
Now try logging into the lobby again.



Ports

List of ports that should be open :

* 27750 (Medieval II - Total War)
* 6667 (IRC)
* 3783 (Voice Chat Port)
* 27900 (Master Server UDP Heartbeat)
* 28900 (Master Server List Request)
* 29900 (GP Connection Manager)
* 29901 (GP Search Manager)
* 13139 (Custom UDP Pings)
* 6515 (Dplay UDP)
* 6500 (Query Port)

More information : www.gamespyarcade.com/sup...alls.shtml


CD Keys:

"LOGIN FAILED - INVALID CD_KEY"

Several reasons why you may get this:

1) You entered the CD key incorrectly
- if so follow this path to re-enter you CD key:
Open Start
Click Run
Type: regedit
Now go to this: "HKEY_LOCAL_MACHINE/SOFTWARE/SEGA/Medieval II Total War

2) You did not enter your CD key in ALL CAPS

3) You have to use 0 (zero) not o or O



If you have Vista (compliments of Lyphe):

Ok, the fix is essentially the same as has been described here already, but I'll expand on it so that you don't miss anything small and easy. I just fixed mine and it now works aok. Here is what to do:

- click on the windows start bubble bottom left
- type 'regedit' in the search bar
- you will now see an icon/link that you can click on above that says 'regedit' - click on this
- you are now inside regedit ... double-click the following:
* HKEY_LOCAL_MACHINE
* SOFTWARE
* Sega
* Medieval II Total War
* Double click and edit the "CDKey" entry (located on the right side pane)
* Update your CD key and you are done.

Make sure when you update your CD key that all your letters are CAPITALIZED. That was the only problem with mine. My CD key was correct, but my letters were small case. I changed that and I was able to log-on no problem after that. Also make sure you use the "shift" key and dont' use your capslock ( per the previous post in here ).

Cheers and hope this helps any Vista people!