For most westerners the word Kremlin conjures up images of slightly sinister Soviet power - the great stone headquarters of an evil empire. The Kremlin, in Moscow, has been at the center of a mighty nation for hundreds of years, through Tsarist, Soviet and now modern political times.
But what a lot of westerners don't realize is that the Kremlin in Moscow is just one of several kremlins. In Russian, the word "kremlin" simply means "fortress" or "citadel". So when we use the word Kremlin to describe the physical seat of the Russian government we're using a purely descriptive word - in a very similar way to how we use "White House". The residency of the US president is literally a white house as well as the White House; the Kremlin is also a Kremlin.
Got that? There are several kremlins to be admired all over western and central Russia - that is, mostly in the European parts of the country that lie to the west of the Ural Mountains. The Kremlin in Moscow is arguably the best preserved, but there are also beautiful, well-preserved ones in Novgorod, Kazan and other cities.
It's generally not a good idea to think of a kremlin as a castle. Although they were built to serve many of the functions of castles, kremlins generally take the form of a walled enclosure of land containing several buildings with different purposes, rather than the more western European format of a single fortress surrounded by curtain walls. A typical Kremlin contains not only military structures, but also historic government and financial centers, as well as several churches.
Part of the reason for the development of this kind of defense was the vulnerability of low-lying, relatively flat Russian cities to attack by fast, cavalry-led armies such as the Mongol Horde of Genghis Khan. A kremlin was a place of refuge, if not for the whole community, then at least for the machinery of church and state. It was also an excellent means of keeping those in positions of secular and religious authority safe from rioting and mob violence, as well as being a symbol of a prince's power and authority.
Because of their variety of purpose, most Russian kremlins are much better preserved than the most western European castles, which quickly went out of fashion with advances in artillery technology. If you're visiting one of the major kremlins of Russia you will typically have a lot to see, from churches and museums to gardens and long, high walls - it's best to allocate a whole day to explore!