When Kirk Douglas smashes through the chapel window near the end of the film, you can clearly see that the window isn't stained glass - it's colored paper. Or perhaps papier-mache of some sort.
The three Viking ships in the film were designed using blueprints for an actual Viking ship salvaged from the water and restored by a Viking museum in Norway. It turned out that the boats built for the film were too accurate, because the modern actors were taller than their historical counterparts. Every second oar hole had to be plugged so the modern men would have room to row with a full oar stroke. Otherwise, they would hit the backs of the oarsmen seated in front of them when pushing the oar handles forward to start each new stroke.