If it's gutted, add 2 windows for cross ventilation. Post some pics, for better advice.

 

Windows aren't the problem. There are plenty of those as you can see in the pics. The problem is when the windows are all closed, like if its raining or during the winter. That's when there's no airflow and it starts to get musty.

https://imgur.com/BfyqdZ4
https://imgur.com/XSBaQec
https://imgur.com/gpw7Mu2

 

Even in the winter, I always leave a window cracked for fresh air. If you don't want to do that, the only other thing that comes to mind is an exhaust fan, in the ceiling.

 

Do you heat it in winter?
Heating in winter fills the room with warm air that holds more moisture and becomes an issue as it leaks into the small attic space.

What is below the floor I see? Is there an open space with dirt below or is that a slab?

What do you plan for the walls and ceiling?

Bud

 

Please post some pictures of how the outside was done.
Just a few of the dozens of things I've seen done wrong by DIY'S and even so called pro's when they try and enclose an old slab patio, no overhang on the roof, no gutters, slab is too close to grade, walls were built leaving the slab sticking out, roof was not built with enough slope because it was not built up on the house roof.