Duel with Mother Nature long enough, and she will always win
This is important to understand. In 2018 humanity has not developed perfect defenses against any natural disaster. Not one. We have improved our survivability, but Mother Nature can always undo our efforts. So long as you're willing to live with this simple fact of nature (e.g., unless you want to bring magic into the mix), read on.
Trenches So long as eruptions are infrequent enough, one of your best defences is a big trench. Especially if that trench has a down-hill grade into the proverbial ocean (Go watch the movie Volcano starring Tommy Lee Jones). Your people will need time to clear the trench after every eruption.
An Absolutely Massive Basalt Breastwork In some ways this is unbelievable, unless there's a natural formation your people are hiding behind. Volcanism is powerful. One of the most powerful of the natural forces. A few rocks won't do. A 6-meter high wall might do... if it's at least 12-meters thick. Your problem is that molten rock is very good at helping solid rock become molten rock. Thus, man-made blockades are a bit hard to swallow. Your people will need to clear the solidified magma on the leading side of the wall after every eruption or each layer of magma will eventually smooth out the leading edge, turning the trailing edge (next to your city) into a
Water Water can help, to a degree. If there's a big old river between you and the volcano, good for you! But given enough magma, rivers (and lakes) can be filled. You'll be dredging. But, a big river is as good as a trench, I say.
Off the top of my head, that's all you can do. Magma has the habit of going where it wants. Please don't forget what I said. There is no proof against volcanic eruption today — much less in the 1300s.
Molot is right that I ignored a great many ways a volcano can kill a person. I consider Magma the easiest to defend against, and remember that my point is "mother nature will always win." If you can't defend yourself against magma, you can't defend yourself against anything else. Given 1300s tech, you can't hermetically seal a city. You might hermetically seal caves, suggesting your city is living in the caves, which would also protect you from falling debris ... except that on a volcano the caves are most likely lava tubes meaning you're either in the direct path of unstoppable magma or you're baking in an oven.
After thinking about it a bit more, the psychology that would bring a community to this point would include the risk of dying from the great power of the volcano. From that perspetive, creating a perfect defense (if such a defense is ever believable) would be to deny the power of your own god, so to speak. It would likely cause a schism in the faith.