Who built Fort Ross and why?

The Russians built Fort Ross. Why? Because, Since the Russia’s Alaskan colonists found it difficult to produce their own food due to the fact that they had short growing season of the far north.
 So, the officials of the Russian-Amer. Company reasoned that a permanent settlement along the more temperate shores of California could be quite useful, as in two reasons... One is it was a good source of food and second, it was a base for exploiting the abundant sea otters in the region.
 In the end, a very large party of Russians (and Aleuts) sailed for California. In which, there they had established Fort Ross (short for Russia) on the coast north of San Francisco.
~Hope this helped :)