Kaer Morhen
Kaer Morhen is an old elven fortress where the witchers are living since a long time now. It is mostly ruined yet, and the only witcher who lives here always is Vesemir. Other witchers stop by from time to time between their missions, mostly in winter, to rest and share their storys.
Long ago the fortress was the target of an assault by humans who wanted to see the witchers dead because they are mutants. Not many witchers escaped this assault and there is only a handful of them left yet.
The laboratory of Kaer Morhen, which is placed in the basement of the fortress, is a place where witchers brew and store their potions. Here are secret alchemical ingredients kept and in old times new witchers were going through the so called "Trial of the Grasses", a procedur which not many witchers survived.
Geralt begins his journey here in Kaer Morhen, he has to help defending the fortress during an attack of the Salamandra and a mysterious mage named Azar Javed, this is the game's tutorial for new players. During the main game Geralt will not go back here, but you can visit Kaer Morhen again in the module "Price of Neutrality".
Vizima
Vizima is the capital of Temeria and King Foltest has his castle, the seat of government, in this town. The most important places in the town are the Trade Quarter, the Temple Quarter, the Royal castle, the Sewers and the Cemetery. There are more places in Vizima but they are not accessible in the game.
Vizima is build on a small island within the Lake Vizima and Old Vizima, a peninsula accessible from Vizima over a dike, is closed and used as a ghetto for non-humans. In the sewers are many Drowned Dead and also a Zeugl has it's home build up here. Well, at least until Geralt arrives...
Vizima itself is a dirty and dissolute city, Geralt will find all kinds of prostitutes here (at least female ones), also many contracts for a witcher and even two important taverns: The New Narakort and The Hairy Bear. The Order of the Flaming Rose has its headquarters here in Vizima too.