In the game's actual final act, you cycle through the four fortresses you explored previously for a total of 20 more defending siege battles. If you haven't upgraded the Orcs you met early in the game--and up until this point, there was no reason to--you have to replace and upgrade your entire retinue of Orcs to match this more powerful invading force. The enemies you face level up with each encounter, so you're also forced into upgrading each castle over and over again, either by building up your current Orc army or finding new fighters and replacing the old. This Sisyphean quest has no corresponding significant characters to keep you company or explain why it's important to tackle the defense missions in the order you do. It's not even clear, exactly, why you want to do them at all.
More than once I felt like giving up on this quest thinking I'd stumbled onto some optional side content that was clearly only made for obsessed completionists. But enduring on, I found that finishing every stage unlocks the final cutscene and credits. It did not feel worth it.