Hallo Robert
It is not so easy for me to explain you in english, what the city of Kassel in state of Hessen has to do with the Duchy of Saxe –Coburg-Gotha and the military, so for better understanding about the history you should first read the info about the Duchy of Saxe –Coburg-Gotha on this page, especially about the military. The page is available in English too.
http://www.deutsche-schutzgebiete.de/herzogtum_sachsen-cobur...The headquarters of the 11th army corps of the Royal Prussian Army (XI Armee Corps) was stationed in Kassel, (Cassel) as I could see in the ranking list of the Royal Prussian Army, for the year 1873. Parts of this army corps have been different units, also the 22.Division,which was also stationed in Cassel. Part of this Division was the 6th Thüringische Landwehr Regiment Nr.95 (means it was the 6th Thüringisches Landwehr Regiment, but the 95th Prussian Landwehr Regiment). The 1.Bataillon of this Regiment was not stationed in Cassel, it was stationed in Gotha, which is not so far away from Großfahner, where your grandfather lived.
You see the connection to Cassel? Your grandfathers could have been soldiers in this 1.Bataillon.
The point that does not fit is, that you say, that both served as pioneers. The 6th Thüringische Landwehr Regiment Nr.95 was not a Pionier Regiment.
But,another unit of the 11th army corps in Cassel was the Hessische Pionier Bataillon Nr.11, stationed in the city of Mainz, at the river Rhine.The Ranking list from 1873 mentions only Mainz, but by googleing in found, that this Bataillon was not just stationed in Mainz, but in Mainz, township KASTEL.
You see the possible connection, Your grandpa kept "CASTLE" in the conversation. CASTLE?,Cassel?,KASTEL? As you wrote, slight shifts of spellings and pronunciation always occur. Kastel would fit with the info of the Pioneers.
A lot of open questions.
I attached parts of the ranking list of 1873 and an old map of Mainz-Kastel from 1893
Klaus