Like all placoderms, except for the giant placoderms like Dunkleosteus and Titanichthys, it had to stay near the bottom because of its heavy head armor.
This image shows a different Doryaspis. But all the species of Doryaspsis are pretty much the same. All the species of Doryaspis have two pectoral spines and one spine right under the mouth in front of their head. Which is why Doryaspsis's mouth probably had to point up.