There are four major modes of traditional Christian scriptural interpretation: literal, moral, anagogical, and typological.

The literal sense is what the text literally means. The moral sense is what the text instructs you to do. The anagogical sense was how the text pertained to the afterlife. The typological sense was the way in which the passage could be interpreted figuratively.

Let's try an example:

Genesis 3:15 (Vulgate): inimicitias ponam inter te et mulierem et semen tuum et semen illius ipsa conteret caput tuum et tu insidiaberis calcaneo eius

I place enmity between you [the serpent] and the woman, and between your seed and her seed; she will crush your head and you will strike her heel.

The literal sense: the snake/devil is cursed, and the snake/devil and humans will always be enemies.

The moral sense: you should always resist the Devil (ie. "crush his head")

The typological sense: the Virgin Mary shall defeat the Devil.

The anagogical sense: the Devil will try to corrupt and poison your soul (ie. "strike your heel"), and you must be watchful.

Here's the important bit: in medieval exegesis all these interpretations are understood to be true at the same time. In other words, your question doesn't quite make sense, because it presupposes a binary (literal vs. metaphorical) which did not, in fact, exist.

So, everyone interpreted and understood the Bible in each of these four ways.

Why do you believe that the bible was 'always interpreted literally' in the middle ages?

Please explain the basis of your question.