Dungeon Upgrades
You can, of course, spend DP to upgrade yourself, or your dungeon in general. Many skills are locked to you, as you're a dungeon core and thus have no use for most of them. However you get unique skills that no other type of creature can get.
You can freely change the layout of one of your rooms, moving objects with no effort, though you can't move them between rooms. As long as you aren't changing the material they're made of or creating new objects, this is entirely free.
Creating objects requires 1 DP per square meter of material used. Even if you don't use the full square meter, the cost is always a minimum of 1 DP to make something new.
Higher quality material increases the cost a little. For instance, making something out of iron is 10 DP per square meter, with a minimum cost of 10 DP. And enchanted items add a 5x multiplier to the cost. So an enchanted iron sword is a minimum of 50 DP.
You unlock access to all skills once you gain the "Humanoid Avatar" skill that allows you to create a proxy body that can be used to physically interact with the world around you. This skill is 100000 DP.
As mentioned in the Domain section, you can spend DP to increase your domain size.
You can use DP to instantly create new rooms. A small room is 10 DP and is only about 10 meters in diameter. A medium room is 50 DP and is about 10 times the size of the small room. And a Large room is 250 DP and 10 times larger than the medium room. You can choose the shape of the room upon creation, but it's material is based on it's surroundings. A room made in a cave will have stone walls, stalagmites and stalactites, etc. Though you can pay 10 DP to change the material to any that you have access to.
Adding a new floor costs 50 DP and doubles in cost every time. So your second floor will cost you 50, your third 100, your fourth 200, fifth 400, etc. A floor comes with a free medium room as it's first room, connecting the two floors. When you make a new floor your core room is moved to the new free room, and your previous core room becomes a Boss Room, which gives a 10% stat buff to monsters located within. Like with Rooms, you can specify the base material of the new floor, and all rooms of that floor will default to that material with no additional cost. Speaking of materials...
Materials
To start with, you don't have access to anything but the natural resources around you. Since you start in a sealed section of a cave in this story, you have access to stone, gravel, dirt, water, etc. You can add a few cosmetic things like maybe there was some marble nearby, if you'd like, but that is all you have to start with.
You gain more materials by absorbing them. So if, for instance, a deer were to somehow end up in your dungeon and die, you could absorb it's corpse and you'd have access to bone, deer meat, blood, and other things found in a deer's body.
This includes gaining access to unprocessed ore, should you find some while expanding, or processed ore should something carrying said metal die in your domain.