You can see quite a bit of gaps allowing light to shine through - that's ok; I think of these gaps as room for the model to develop personality. Let's get those gaps filled and see what becomes of our mysterious well.
Normally, I always add a bit of water and a bit of paint to prepare the spackle for application. When the spackle is wet, it is easier to work. Adding a little bit of water, the spackle still holds it shape and has some spring to it; good for vertical fills and connecting two points. Adding more water creates a slurry, good for when you want gravity to work for you and have the spackle settle into low areas. I advise starting with a bit of water, as I almost always do, because you will be wetting your tools and fingers as you go, so the spackle will get wetter as you need it o without turning to soup.
Now, get a sponge or some foam. I use the foam that I pulled out of my miniatures carriers. You can also use the foam that is packed in most miniatures when they ship. Grab a cup of water. Wear some clothes you don’t mind getting splashed (just in case) and put down some plastic or paper. With the wet sponge, wipe across the spackle-covered stones with medium pressure. This will smooth the spackle, push it into the lower crevices, and remove it from the upper part of the stone, leaving it exposed. Again, you will wash as much or as little to get your desired look. Keep the sponge wet and continuously clean it out with fresh water so you are not just smearing spackle everywhere. I used medium pressure and I did the entire inside well once, let it dry, went back to that night and wiped it again to get my final look that I liked.
I wanted a look where the mortar has worn away and the only remaining mortar holds the stones, for only a few more years. If I didn't remove enough spackle, I would wash it again for a third time. If I remove too much, I have to add some spackle and then wash again. I like the inside so I repeat the same steps for the outside. This is what it looks like after the outside is done: