Alright, Tibius the wise asked me to send a letter to a specific city/town. I sent a hero alone with the letter and my hero is just next to the city. Now, I'm not sure what happens now. Should I walk "into the city" (as if I want to attack it?). Becuase there's no "pass" option with the city name.

Walk into the city, it will complete the quest, and the gold will be placed into your cities treasury.

Whilst under the Dark Aura, you cannot instigate attacks on other players, it will tell you that you cannot do so under Kresps protection.

Thanks Scrod!

I got my 100 gold!

LOL the person who I was sent to send the letter to attacked my Heroine. How do I communicate with him? is there a message/chat system?

Yes there is.
When you login to the game you are automatically redirected to your inbox.
To write a new message click "New message" link at the top.

Also if you have any visible object on the map of the person you want to contact, click on it holding Shift button with your left mouse button.
You will see a description of the object and hyperlinked owner.
Click on the link and scroll down the info panel, you'll see a quick message preset. Enter the text and hit Send.

Thanks Admin!

I sent him a letter and I have to say it was the most RPing experience in awhile. It was formal and I demanded an apology otherwise his hostile attempt to my messanger would be considered an act of war.

Worse thing happened to me: a new player spawned right near my towns, settled a village and then immediately attacked and killed my heroes used for goods transportation. Of course I eliminated that warmonger but still it was annoying...