How do I get inside this door?

Albatr055

Grond0

For that sort of detail, why not use a walkthrough, e.g.
https://lilura1.blogspot.com/2016/04/Siege-of-Dragonspear-Walkthrough-Part-I.html

Albatr055

OK I figured it out.

I missed the loot from Zialtar who drops the key I need.