“Capital” and “capitol” are very easily (and very often) confused. “Capitol” is only and ever and always a building. Think of the round O-shape of a dome. Think of stone, of doors, of windows. All those things echo the “O” in “capitol.” One is bound to work for you as a mnemonic. If this word refers to a specific building, it will be capitalized as a proper noun or proper adjective. “Protesters in Washington, D.C. congregated on the Capitol steps.” However, it can just as easily be a common noun: “While the class was in the capital, they toured the capitol and other important sites”.
“Capital” is never a building. (This is one of the few times I feel safe using that adverb.) The capital can be a city (Madison is the capital of Wisconsin). It can be a letter (city names begin with a capital letter). It can mean “the center of a specific activity or industry” (Hollywood has been called the entertainment capital of the world). In the UK and countries where UK English is the usual, “capital” can mean “excellent,” as it does in the heading for this post.
For some reason, this pairing has been turning up all over of late in various grammar-related places. I’m far from the only one to have addressed it. Here is a link to the Grammarist article, which I think is one of the best.