Building a world after the fact.
Part 1 - The LandsBuilding a world while writing by the seat of your pants is not for the faint-hearted. Some of it can be hand-waved if the characters aren't exploring or travelling much. Once they leave a small section of a city, things start to grow. If you do wind up creating a world whole cloth on short notice, here's what I learned.
Part 2 - Militaries
Part 3 - Elves
Part 4 - Architecture
Part 5 - Magic
Part 6 - Wrap Up begins now.
First, when in doubt, choose a part of the Earth that resembles what you want. Google Maps helps here. Don't worry about the scale - you just want something to refer to. I used the lower mainland of British Columbia, then changed the distances to what I needed. The various kingdoms can be as large or small as you want, too; counties may work well as kingdoms, especially if you zoom in close enough. Use Earth cultures as a base to give characters names that don't sound like a cat walked across a keyboard.
Have in mind a rough idea of what technologies are available. The traditional fantasy world needs iron smelting at a minimum to produce the weapons and armour characters use. The nameless guards will have near-identical arms and armour, mainly because of economics of scale. Keep notes on what you decide; if the town guard in the king's city wear chain mail and carry short swords while keeping the peace, then a reader will notice if a guardsman is wearing leather or plate. Not that the guardsman can't, but he'll need a reason.
Non-humans should have their own histories. Look at how humanity developed on Earth. Why shouldn't elves and dwarves have the same diversity? Feel free to change things up, too. There is no reason for every elf to be a Legolas clone. Using names based on foreign languages may shake things up and give a new perspective.
Buildings are going to be functional. The technologies used today require iron refining that wasn't available in the Renaissance, though the ideas were there. Stone and wood will be the main construction materials, though bricks are a possibility in some areas. At the same time, once magic gets involved, figure out how that will change building design. Ultimately, a house will need walls, a door, and a roof, and even magic can't change that.
Figure out, at least roughly, how magic works. You don't need to work out every last detail, but the general nature should be known. Do the mages work with the classic elements? Or do they shape raw mana with their mind? Ideally, this is worked out before the mage casts the first spell, but sometimes, the spell gets out before the nature of magic is ready.
Finally, don't stress over the details. Not everywhere needs to be fleshed out. That inn the characters stay at one night can feel more real by adding description. Its history isn't as important as what is happening at the moment. Keep open to ideas that pop up. Dwarven canals to ship raw and refined ore? If that fits your concept of your dwarves, why not? Or maybe the elves are the technoligical types instead, with canals to bring their armada in from the ocean. It's your world. Have fun with it.