So you decided on fortifying! To add some defenses. Good decision but, Hey, you ask, I don't know where to start!
Fear not grasshopper. I will tell you what you need.
First off; you have to decide what defenses you need. Will your map be a city with a ring wall against raiders, and to controll import/export tariffs? Will it be a fortified city that could stand a seige? Will it be a fortress in it's own right?
This essay will deal with all those issues, but we'll start out with the basics, then look at the fortress.
The basics of fortification comes down to defensivness versus investment. Walls, towers and fortressess are very expensive. The project can become on the order of pyramid building in its scale.
Carlsborg fortress in sweden used up 250 000 metric tons of limestone for its land front and the 5 kilometer walls covering the waterfronts required enough earth to build 20 pyramids. In a fantasy world, much of this can be fudged, so no problem, a few lines on a map won't cost you anything, but keep this in mind. Use an engineers perspective rather than the designers. The designer adds until there's noting more to add. An engineer will take away until there's nothin more to take away.
Now, let's talk about what a wall is and why you'll want one. A wall is a man made obstacle to prevent someone from accessing a place. In fortification terms, it should keep the enemy outside your defended area and stack the odds against him. Common planning says an enemy force must outnumber the defenders by at least three to one for a successful siege or assault on a fortified target.
The fortifications are also platforms where your ranged weapons can work against the enemy and protect the target from the enemies ranged weapons. The medieval walls were build high and thin, with crenelation at the top to give the archers protection. Towers gives the archers added range and fields of fire. A wall must be in the fields of fire from as many different places at once as possible, but the archer at the top can't fire straight down, so the towers portruded from the walls (called hanging) to give fields of fire along the walls.