Indeed, one of the most common misconceptions about WW2 is that it was a highly mechanized affair, particuarly Germany with blitzkrieg; Hollywood has hardly helped in that regard as tanks tend to be flashier. None of the belligerents in WW2 ever had more than 15-20% of their formations mechanized or motorized. The rest were infantry which had to get where it was going the same way Roman legionnaires did, marching on foot. In the German army as with most others supplies and artillery for the infantry was horse-drawn, Germany used 2.75 million horses during WW2, more than they had used in WW1. The US and the Commonwealth were unique in that their supplies were fully motorized, their infantry however wasn’t. The supplies and artillery in US and Commonwealth infantry divisions may have been moved by trucks, but the poor bloody infantry still got to march wherever they were going.
The plot premise is that the eponymous guns are denying the Navy access to a strait which can be used at night to evacuate British and Empire troops fighting on the island of Kheros (compound of Kos and Leros) before they are overwhelmed. Of course there were no such guns but it is anchored in the Dodecanese campaign, after a fashion.