A “capitol” is almost always a building. Cities which serve as seats of government are capitals spelled with an A in the last syllable, as are most other uses of the word as a common noun. The only exceptions are place names alluding to capitol buildings in some way or other, like “Capitol Hill” in DC, Denver, or Seattle (the latter named either after the hill in Denver or in hopes of attracting the Washington State capitol building). Would it help to remember that Congress with an O meets in the Capitol with another O?
BUY THE BOOK!