You are correct that 'some' in this instance is another way of saying 'of about 90 stories high'. Stories is the plural of story, and in this context it means levels of a building. A building with 10 stories - levels or floors - would be called a ten story building.

Ninety storeys (US: stories) high means the same height as a building with 90 floors. The Eiffel Tower of course has no floors -- it's just a reference to its height.

You are correct in understand that "some" here means something along the lines of "about". In fact, I'm pretty sure this usage may have come from a phrase like "something along the lines of" or "something close to". A "storey" is a measurement of building height. It usually refers to the number of floors a building has with smaller buildings. A good estimate is approximately 10 ft (3 meters) per storey, but it is my understanding that this is a very rough estimate and the "storey" is a rather vague unit of measurement. 90 stories makes for a very tall building, regardless of the specificity though. Hope this helps :) (One last minor thing: It's spelled grammar, not grammer ;) )