What style are you going for? for strong medieval fortresses, I feel like stone, stone bricks, and cobble are best. A bit of spruce throws in some dark yet saturated color, which contrasts nicely with the bland, light colored stone and cobble. Another tip i have is to mix smooth stone and cobble randomly for flat areas and use stone bricks and stone slabs for supports/pillars. Afterward having spruce pillars topped with stone brick stairs on the side of the wall every two blocks can add detail. Lastly, I GREATLY encourage you to start small, and expand later. It will make tweaks much easier later on, and having a huge empty space in your base isn't pleasant.
Oh and another thing, I think that some people do overboard with detail. In regalia I feel like all the different block types make the whole thing hard to look at. Don't worry if you have areas that are simply empty walls, its better in my opinion the have empty walls then not even being able to look at your awesome looking towers.