Like many doom/quake clones (before ID released the source) I expect openage to be supplied without the artwork, you will need an original CD/image for that.
Yep, the assets don't ship with the codebase - you need to provide your own original AoE II (or AoE II HD) installation.
Where does a designer stand if they want to make the missing resources so that the whole game can be shipped for free?