There were four small openings around the base of the castle. I filled them in and added a gun for each one. I only needed the barrels, I saved the chassis for other purposes.
Mostly I used PaK 75 anti-tank gun barrels, since they were hard hitting, were in service late in the war and I had some surplus ones.
I simply stacked one bit of sheet styrene on top on another with a little hole in the center to create the step back effect. Many bunkers had metal protective guards that went right up to the gun barrels for greater protection.
Once I started work on the castle, I realized they had the big front door, but no actual way to get there. So I put a giant door at the back of the mountain part. My feeling is that the interior is hollow and secret weapons research happens inside. That way large vehicles and missiles can come and go with ease. To better protect the door, I used a 20mm gun with machine gun combination in the bunker near the door. Higher up there was a hole, so the 75mm PaK went there. The door is from a railway box car I got at the recent train show. I simply cut it out of the side of the boxcar and instant doorway.
The arch is for a little drawer originally designed to hold the figures that came with the set. I painted it to look like an observation port for the gun above it. You can see the outline of the drawer, still opens and closes.