This tutorial will show you how to make a well.

First, you want to make the well walls. Make a brush about the size you want your well to be.




You're going to want to make your well a cylinder, so go to Patch > Primitives > Cylinder. (The outside of the well does not need to be as tall as the inside of the well.)




Now you need the inside of the well. Select what you currently have and duplicate it (Spacebar). Now invert it by going to Patch > Invert (Control + I) and resize and move it so that it is smaller than the outer wall by 16 units on the X and Y axes.



You will also want to add more depth to the well so stretch the interior cylinder down on the Z axis. Once you have done that, select the brush, open the surface inspector (hit "S"), and hit "natural" so that the cylinder's texture is not stretched.



Now, you're probably going to want a mantle for the well. So copy your exterior wall and resize it so that it is eight units high and four units longer on each side (eight total) on both the X and Y axes. (Again, open the surface inspector and select "natural")



Duplicate what you have made in the last step and invert it. Now resize and move it so that is about four units on each side (eight total) smaller on the X and Y axes than the interior of the well.



Now you need to make a top and bottom for the mantle. Create a brush and make it a simple curve patch by going to Patch > Simple Curve Patch. I am going to use a 3x3 patch to create the top and bottom to my patch.




Move the vertices (hit "V") as the following image shows.







Now duplicate that layer and rotate it using the z-axis rotate button. Move the duplicated simple curve patche to their corresponding location on the well's mantle. Do this until the top of the well's mantle is covered. Once you have done this select the top pieces of the mantle open the surface inspector and hit either "natural" or "cap".



Now you need the bottom of the mantle. Select the curves used for the top of the mantle and duplicate them. Use the x-axis rotate button to flip them over and move the flipped simple curve patches to the proper location.



You now need a bottom for the well. I copied the well's exterior wall and moved it over to the side. Have the newly copied cylinder selected and cap it by going to Patch > Cap (Shift + C).



Select all but the top newly created curves and delete them. Now move the remaining curve to the bottom of the well and resize it so that it is not to big nor too small for the well (basically, you don't want any gaps).

You will have to move the vertices of the terrain around the well so that there are no gaps between the terrain and the well.

To move the vertices of the terrain, have the terrain you wish to modify selected, hit "V", and select the vertices you want to move by holding Left Mouse and dragging your mouse over them on the 2D screen. Now hold Alt and Left Mouse and drag the vertices you have selected so that there is no gap. You may have to move more than one vertice on a single simple terrain patch.

This is what it should look like in-game.




Here is what I had at the end of this tutorial (excluding .GSC, .CSV, and .FF).
http://jamesharris92.googlepages.com/well.map
http://jamesharris92.googlepages.com/mp_well.map