Removing rocks with tunnelers to create passages

It might seem that impassable terrain is, well, impassable, and a blocked path cannot be unblocked except in map editor. Recently I have discovered that this is not always the case. Even during the game it is possible to do limited terraforming in a way that, depending on the map design, can dramatically affect the gameplay.

Here are the facts that make this possible:

• When you place a building, the area below it and the surrounding rim about 3 tiles wide is converted to "ground" terrain;
  
• This affects passable terrains such as stones and grass, but also rocks;
  
• Rocks are commonly used by mapmakers as a lazy way to block passage.
  

As a consequence, cheap buildings like woodcutters can be built next to rocks in order to remove them.

Now, you wouldn't normally be interested in clearing defensive terrain around your own base. It would be much more useful to do this to your opponents in order to bypass their defenses. The problem is, you can't place buildings too close to the enemy. There's one exception to this rule though... the tunnel entrance.

That's right, the tunneler's tunnel entrance is a building like any other; it is even listed with the buildings in the PDF manual. This means that it can be deleted or destroyed by enemy troops and, most importantly, it also converts surrounding rock terrain to ground.

Since the tunnel entrance appears immediately when you order the tunneler to dig, the terraforming effect is also immediate. It doesn't even matter if the tunneler actually reaches the entrance, so you can place it right next to the enemy walls.

You can place the tunnel entrance even in an inaccessible place. In this case it will immediately disappear and the tunneler won't be used up, but the terraforming effect will still happen. It works even across walls, if the wall is less than three tiles thick.

To provide an example of using this technique, I've chosen a siege map called "User Created Map - Castillo de Loarre", which comes with Stronghold 1.4. The outer layer of the castle walls is almost completely surrounded by rocks, except a couple of short stretches around the gates, which are heavily defended. Further complicating the matters is the fact that you cannot build siege machines.

Fig. 0. General layout of the castle and my attacking troops. For the purpose of this instruction, only tunnelers are important.

Fortunately, there's a sizeable gap between the inner walls and the left edge of the outer wall. A wide band of rocks terrain makes this gap impassable... or so the mapmaker thought.

Fig. 1. Close-up view of the gap not protected by walls. The spot where the passage will be created is marked with a yellow dot both on the main view and the minimap.

I selected the tunnelers and ordered them to dig next to the inner edge of the rocks band. Notice how the space behind the band is still inaccessible, but some rocks have disappeared. I repeated the trick a couple more times until enough rocks were cleared. Since the last tunnel entrance became accessible, it didn't disappear; the tunneler started running towards it and got shot down by enemy archers. Too bad.

Fig. 2. Progression of clearing the rocks.

The passage allowed me to bypass the outer layer of the walls completely. Since the inner wall was lined by rocks too, I had to repeat the tunnel trick (in retrospect, I could save a tunneler by doing it before opening the passage).

Fig. 3. Inner wall about to be torn down by pikemen.

After my crossbowmen cleared the inner court, they were able to shoot at the defenders of the outer wall from the higher ground, while protected by towers and shields. Castle taken, and with far fewer casualties than a frontal assault would bring.

Here are my concluding thoughts:

1. Is this cheating? I think not. This trick doesn't exploit a programming error (like e. g. teleporting tunnelers do). This is just an unusual combination of known game features.
   
2. Tunnelers are actually useful! Well, at least on some maps.
   
3. Dear mapmakers, rocks are passable. Please don't be lazy; use height differences and/or boulders.
   

The attachment contains the images linked above, in case the hosting ever takes them down.

tunneler-trick.zip

Really thorough and well-organised tutorial! Very well done,?@odisseus! Topic has been stickied!