The Siege Engine has two spots that you’re able to hit to deal damage.
For some easy, low-damage attacks, hit the facewith your sword.
This isn’t a weakspot, but it will deal some damage.

This capsule is a weakspot, so slash it with your sword to deal plenty of damage.
As always, there are plenty of other items that you might use to make this fight easier.
That covers how to defeat the Siege Engine in Tunic.

If you’re unsure what to do after defeating this boss, take a look at ourcomplete Tunic walkthrough.