Lots of good resources out there like the Cartographer's Guild or Dyson Logo's stuff on Patreon. But for those who like to do things themselves...can discuss tips and tricks here a bit too.
I know when I start a new town or city map I consider three things primarily. What the environment is like in my world where I am placing it. Water source for both potable and sewage disposal. Terrain and contours. When I go to draw it, I generally start with the water bodies if any. Next I will draw contours at 20, 50, or 100 intervals depending on scale and whim. Then I lightly put in the main roads and think along the lines of market area, stockyards...and how the place built up from its founding. Are defenses needed and if so, from what type of dangers? A massive city wall and gates is not going to do a lot vs purple worms and may only be marginally effective against flying threats.
Dungeon maps...I look at the ecology of the dungeon. Things like food chain, water sources, waste disposal, and most of all...purpose. I see alot of maps being done out there that just don't take some of this into account very well which sort of puts me off from using them. Some good ideas there though. But purpose also gets back to size of a dungeon. To me an Encounter or Lair is 1-5 rooms. A small dungeon / stronghold is 4-12 rooms. It just goes from there. I see far too many comments calling dungeons with more than twenty rooms mega dungeons or some silliness like that when in fact, they are more true to a real delve than the tiny one-off things being touted as dungeons these days. But I digress.
Scale and whether to draw the map isometric or plan view is something to consider as well. Iso has some good strengths to it but is difficult for a lot of folks to do well. Plan view is easier but does generally leads to the map maker thinking in only two dimensions instead of three which is one of Iso's strengths.
Putting yourself in the place of your dungeon's owner/creator is also recommended. Don't simply look at the creation process from the rule of cool but do so effectively from the perspective of the ones depending on its defenses and layout for protection and uses. It does no good to have a massive fortress if the master is easily cut off from vital supplies like food, water, and medicines...or troops cannot easily get to their armory...etc.
Are the halls large enough for the creatures within to effectively fight in? A 10' hall with standard ceiling will reduce your polearm and two handed sword weilders to stabbing only, not having enough room to properly swing their weapons. An ogre is not going to fit through a 5' wide tunnel much less be able to do more than grab and squeeze something if he manages even that.
I'll leave things at this for now and see what everyone else looks at on this topic. :)
I have a FB page for all my maps:
https://www.facebook.com/TheMadCartographersCartography/
Settlement Defenses. I see so many maps these days by some really good cartographers...but they are in a medieval mindset and not a hostile environment mindset it seems. Few, if any, really take into account town defense. Sure, a small thorp of less than a dozen buildings is not going to be able to afford a massive stone wall and armed guards. They don't have to. A single common building is more likely to be fortified similar to a manor-fort and/or hedgerows created around the boundary of the fields to protect the livestock from wild animals as well as slowing down would be attackers. Villages of 50 or so buildings are more likely to have wooden pallisades are a small keep built by the local feudal lord (if any) or church and again, hedgerows, irrigation ditches, and such through the fields around the village...and this just works its way up. With the prevalence of monsters, brigands, and goblinoids running rampant through the campaign world, only the suicidally inclined are likely to live in a settlement without some sort of easily defendable refuge for the populace in case of attack.
Then lets get into terrain. Only a flaming IDIOT is going to build a settlement or stronghold in a location that allows the enemy to get directly or even indirectly above them and any walls that might be serving as a main defense. It is for these reasons that ancient castles were built on the HIGHEST terrain possible or on ground with few direct paths of assault. While all these fantasy cities up against cliff faces or in valleys look cool...from a military and tactical standpoint they are nothing more than sitting ducks waiting for destruction. But there are walls and towers along the cliff above the town as well to prevent that. Yes...ISOLATED towers and walls with only limited ability to receive timely reinforcements and that are subject to well organized sneak attacks and infiltration with no one below realizing it until after the fireballs start raining from above.
Then there are those town maps drawn with forests and fields right up against the town walls. Again...someone being suicidally stupid. No town is going to allow that since the trees serve as bridges onto the town walls. The tall crops of fields or brush and trees serve to hide approaching enemy troops from lookouts manning the towers and walls, not to mention give partial or complete cover against defensive missile fire from the town. The reality is that crops are unlikely to be allowed closer than bowshot to the walls and forests and stands of trees/brush are going to be cut back to catapult and ballista range. Not only for defensive fire but to also provide clear ground for town dwellers to grow crops to support the city. New buildings are also likely to be discouraged from being built closer than bowshot due to the cover issue until additional defenses are constructed farther out; such as a palisade wall and towers around the previously clear range/fields...and additional clear range created beyond the new walls.
Defenses are expensive yes, meaning that the defenders are more likely to utilize the cheap options as much as possible such as the clearing (which also generates revenues in the long run) rather than building more than necessary. Again this folds back into terrain considerations when building the settlement.
Ports are another thing commonly poorly mapped. Ships do not turn on a dime, especially sail powered ships. And then there are issues such as draft and current to consider...which are nominally ignored. Especially on rivers. River bottoms and sediments are not stagnant and are constantly changing season to season with suspended sediments dropping out in areas that the current slows, such as eddies, wide sections of river that the water goes placid, and created anchorages/harbors which will require constant dregging. River mouths are especially an issue for this due to the sedimentary deposits known to create delta regions as well as the tidal bores and breakwater zones that are often so violent that they sink ships and small vessels.
Then there are floodways and flood plain issues to deal with. Water tends to want to go in a straight path during flooding and will flow accordingly without some significant geological feature preventing it or keeping it from cutting new channels to get where it wants to go. No one with half a brain is going to build a permanent settlement in such a place...the first flood that submerges the town or starts washing away buildings convincing them that relocating is a more profitable and survivable idea than rebuilding. Dikes and levees can be used of course, but again we get back to cost, available manpower for construction, and overall defensibility. A simple dig spell by an enemy earth mage or a creature tunneling through the levee allowing the town to be drowned is a weakness no tactically sane person is going to be willing to tolerate.
Sure, many towns were established at cross roads, river fords, and other areas noted for merchant traffic and trade routes. But even the residents of those spurious settlements are going to be looking at defense and personal safety for them and their families. Especially since such towns become very profitable targets for bands of thieves, mercenaries, and pillaging humanoids. Cycling back through the above issues.
So when you start laying out your town in the mental mindscape, keep defense and upkeep in mind. While a medieval type setting it is not, in fact, a medieval europe...it is a hostile environment with enemies on all sides and danger ever present making protection a priority in thinking.