This tutorial is quite involved, so gather ye snacks while ye may.
First of all, think of what a crumbling building or castle looks like. There are pieces missing from the walls, overgrowth, stones litter the ground, some pieces may be shored up with bracing, there are holes where possibly a cannon ball or battering ram went through. With that in mind, let's get started.
| First let's take apart my castle. Here is the castle wireframe from top. There's a tower, four main walls, two inner walls, and some stones just layin' around. |
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| Let's start with the front wall, the one where the gate used to be. |
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| Here is the wall in wireframe view. Notice that it is actually made up of 3 terrains. |
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| Left wall. |
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| In wireframe view (flipped, sorry about that). |  |
| Right wall. |
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| In wireframe view. |
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| Back wall. |
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| In wireframe view (flipped again). |
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| Tower, draft & final version. |
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| In wireframe view (final version). |  |
| Turret & main tower in wireframe view from top. | 
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| OK, let's build the front wall & the tower, the rest you can figure out from there. | |