I usually go to the homepage, go to "Buy LEGO," and that gives you a list of main categories (sets, parts, minifigs, etc).
If you are looking for already made castle figs, then go to Minifigs.
It will have this long list of different themes, find Castle, and the right sub theme. The next paragraph explains the next steps (it is the same with Minifigs as it is with Parts)
If you are looking for parts of a minifig, or regular LEGO pieces (like the rock/plant/tower/wall pieces you need), go to the category "Parts." That will also have a list, but I usually search for what I want. Since I don't know the official names of the Lego piece I am looking for, I never touch the actual search bar. I go to the slight right of it, where there are 2 of those tab thingies. Click on the first one (the one that says "All Part Categories), and you can scroll and narrow your search down by different parts. (Ex. Brick, brick modified, tile, plant, etc). Then I go to the other tab thingy (All Colors), and select the color I want. Then I press search. It will give me a list of pieces in that category, and you can scroll down till you find the one you are looking for. Underneath the picture of the piece you're looking for, it will have a link that says the color name, and in parentheses list how many pieces they have of it. Click that link, and it will take you to a long list of the parts you are looking for, separated by different stores. Near the top, you can set how to list the parts (any condition [you can change that to "New or "Used"], Per Page [you can change how much it lists per page], and Best Price [lowest price, highest price, etc]). Once you find a store that has the amount you need of that piece, you can see the price per piece on the right. Usually you can tell if the store is in the USA or in some other country by what the price on bold says (US, Eur, Idr, etc). When you click the store name link, it will take you to there store, and the part you are looking for. If they don't live close enough (Ex. you live in US, they live in Asia), I find someone else, since overseas shipping is expensive (unless they have a crazy deal). Then I go to the stores search list on the left, and see if they have any other pieces I need.
Then etc etc for other pieces.
Hope this helps, and is not to confusing!