Castle Koopa is the castle that appears in a day with Bowser Jr. Two Koopas for a throne Part 2.
When Lemmy and Iggy are giving their side of the story, when Bowser, Peach, and Grandpa Kamek were talking about some sort of secret truth that has not been told, Bowser says in frustration, "If only that incident at Castle Koopa had never happened, None of us would be in that stupid mess!" And angrily threw a pillow at the plant that Lemmy and Iggy were hiding behind. Bowser Jr. decided to look it up on Koopapedia where they found out about Morton Koopa's death..
- As shown briefly on Koopapedia(if you look very closely) you can read the history of it which shows that it was built as a fortress to hide Princess Peach since Mario could easily take down an airship and rescue Peach.
- Morton Koopa Sr was involved in a false Bowser Mario battle as it said in the section 4.1 Morton Koopa(False Bowser)/Mario Battle
- Morton Koopa Sr might have died there since section 4.2 said "Morton. Koopa's death.
- There were two Bowser/Mario battles there. The 1st one was mentioned in section 4.3 and the other one was mentioned in section 4.4
- In section 5 Called "Attempts to demolish it there was two Grand Toad Attacks mentioned in section 5.1 and section 5.2.
- The two toads that appear on a base surrounded by lava might have been part of the grand toad attack.
- To explain how they got there in the first place would be that the base they were on must have drifted off to a place where they can't escape.