The first step is to "draw" on the table the map of the castle using styrofoam pieces.
At this moment, you must be carefull not to be to precise: 90 degrees angles and perfectly straight walls were usually not a specialty of medieval caste builders.

Now we are going to cut the stones that will represent the remnants of the walls.
In order to do this, we are first going to cut the styrofoam using a hot wire foam cutter in its length, in order to separate the two lengths of stones.
Since these two parts were cut from the same part, they will be perfect match.

Now we will apply the same method to the upper part of both styrofoam strips. You can also notice that we voluntarily digged a little bit deeper at the places were the stones are supposed to be joined.

Repeat this operation as many times as necessary on all the walls of the castle. For the round towers, the technic is exactly the same except that the towers are well ... round.

Here is the final result. One step that is not shown on these pictures is the texturing by water diluted Wood Polyfilla, both on the ground and walls.

To paint this table, we applied a method we came to call Random mix: On a still wet base of brown paint we added some dots of green, yellow and blue.

with very large brushes we then worked on this added dots of paint to make progressive transitions with the brown base.
this was done quite brutally, as you can see on the picture.

For the ruins of the walls, two simples drybrushes of grey on a black base are more than enough.

Here is the finished table.
A few additional finishing steps that we could have performed are the following: green shades on the walls, some static grass on the ground, etc.