Attracting People
Based on what information you posted, I can give you some ideas of how to attract people to your site. This is partially based on how people were attracted to new areas historically. I am assuming you're using the pseudo-feudal and pseudo-historical setting that D&D is generally set in. This all assumes that typical citizens are free peasants who can travel to a new area and are not serfs tied to the land who would need approval from their lord to leave.
First off, you'll want to build some kind of a road to the area so people can get there. This doesn't have to be anything complicated, just a path wide enough for carts that has been cleared of trees, underbrush, and large rocks. It should follow the easiest terrain along a reasonably short path. That's how roads basically get built. A dirt road is just a path that's been beaten down well.
You're going to need to be able to protect the people. (If there was a dungeon, it's probably an area known for dangerous monsters, so you need to overcome fear.) A fortress of some kind would go a long way toward that. It doesn't have to be anything complicated. You're building a tower already. You can build a wooden wall around it so that you have a secured area for people to flee to in case of attack. The easiest way is the old moat and bailey construction used in the British Isles. You dig a dry moat, maybe 10' deep. You use that dirt to build up a hill in the center. You put your keep on top of the hill, put a wall at the inner edge of the moat. (Contrary to Hollywood, most moats are dry. They're usually only wet if you have a stream or river you can divert through the moat.)
To attract people to the area, you have to give them some reason to want to come. I'm assuming the land is arable and that you're going to attract farmers. Unless there's heavy mining or timber resources in the area which make it worthwhile to import in all your food, you're going to have to start from the ground up with farming.
If you are willing to give people land, then that will be a good attraction. If you have extra liquid capital, you can offer a really great deal like each family who moves in gets a plot of land, a cow, and some building supplies for their first house. In modern context, that would be like giving someone land and a free truck to go with it. You may also want to do things like buy a bunch of chickens, pigs, goats, and other smaller animals both for your manor and for giving out to people to help attract them.
If you want to start attracting a village, another good thing would be to buy a team of oxen and a good plow. Then let the villagers use it. Typical medieval villages would often share a single plowing team because the cost was just too much for any one farmer.
If you have good money supplies, work with a religion you like to set up a church as part of the manor. This will help convince people it's a good place to live.
You'll want to advertise in cities that you are making these offers, including whatever gifts people get if they're willing to settle. Set minimum standards, like you want married couples or families and not single men. (For the farms at least, you may want general workers.) You don't want to advertise in other farming villages because 1) Those people already have farms; and 2) You'll annoy other lords if you steal their peasants.
Don't offer cash incentives. That works in modern times where there are places to spend the money. If you're out multiple kilometers from the main road, there are no shops. You want to offer people the stuff they need like tools, animals, and building supplies. It may cost you a bit to truck it all in, but it will be more attractive to a farmer than cash.
When you build the tower, you're going to have to import workers. Once the tower is built, you can offer them some land or other options as well to get some to stay around. A master mason won't stay unless you're planning on more buildings, but general laborers might, particularly if they can get some land rights.
I highly recommend the
"A Magical Medieval Society- Western Europe" if you plan on going anywhere with this setup over time. There are rules for how a manor is set up, how much profit it generates, and how it relates to the larger kingdom. There are also detailed rules on construction that take into account bringing in stone and other important factors. It's a well researched book.
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