Der Eisendrache has two important buildable items that require you to find a workbench: the Ragnarok DG-4 wonder weapon and the Shield. There are three such crafting locations on the map, but getting to all of them is easier said than done.
You need the items to complete the Der Eisendrache Easter Egg.
The ideal workbench locations for you depend on which item you need to build and where is your training spot. You should always build the shield as close to your training spot as possible but the Ragnarok DG-4 as far as possible.
The first Der Eisendrache workbench can be found in the Wundersphere Hallway above the Double-Tap 2 perk machine. This is relatively a safe spot for you to build your shield but it might be somewhat inconvenient if you are playing BO3 Zombies solo and where you decide to train.
For example, if you are training in Bastion, this is a good spot to build your shield due to its proximity but not for the Ragnarok DG-4.
There are also not many Wallbuys around this area so it might get somewhat tricky during the higher rounds.
You can locate another workbench in the Pyramid room right across from the Dragon’s Head. This workbench location is safer than the rest as not as many zombies spawn here and you have plenty of room for movement if they do. However, it can be somewhat of a challenge to get here from your training spot.
The last workbench in Der Eisendrache is located in the Mission Control area which happens to be inside the Church. This is the best location for you to build the shield if your strategy for Black Ops 3 Zombies relies heavily on camping.
Even though getting in and out of the Church is slightly tougher compared to the Pyramid, it is easier to reach, especially if you are training in the Courtyard.