Knights attack portals, but blood moon is too strong.

[New Lands] It’s day 15, island 3. I sent two knights and their squires to attack the second portal on one side of the castle. While they are there, a blood moon happens. During the blood moon, they both return, but soon after, a breeder and a hoarde of greed have broken through the wall on the opposite side of my castle, and I have no way to defeat it.

How do I go about attacking portals without having this happen? Is it just bad timing I experienced?