Let's say, hypothetically, that the Romans had constructed a perfectly symmetrical wall, with the same defenses on both sides. If they were attacked from the inside, they could hold their side- if attacked from without, they could, I suppose, all crowd inside the wall and defend from there. However, what would they do if attacked from both sides, as happened during the battle? You can't defend both sides of one wall.

Speaking more towards orientation, you want a wall to be too tall for the enemy to easily climb over, but you also want your men to be able to look over the wall and use ranged weapons or repel assaults. With large stone fortifications, this takes the form of a chemin de ronde - basically, a walkway behind the parapet - together with stairs leading up to the top. You do not want stairs on the outside of your wall.

In the Romans' case, their fortifications would likely have consisted of an earthen embankment with a wooden wall at the top with towers behind it, deep open trenches immediately in front of the wall, then shallower trenches filled with spikes and covered with brush, deep holes dug into the ground with spikes placed at the base, areas with iron spikes sticking right out of the ground, etc. These defenses would have extended out several hundred meters. You don't want all that inside your fort, what with everyone falling into pits and being impaled. In addition, these are all designed to impede access to the wall, which is the last thing you want on the inside, when you're going to be sending every man available to defend your side of the wall.

So, what does it mean for a wall to be oriented outwards versus inwards? You're going to have all of your spikes and trenches on the outside, and your wall will have as steep a face as you can make, while on the inside there will be an area for your troops just behind the parapet, as well as towers for ranged weapons. Walls really don't work both ways, which is why a breach in the lines is such a big issue for defenders.