Is this self defence?
German self-defence laws may be found in within Chapter 2, Title 4 of the German Criminal Code at sections 32 to 35:
Section 32 Self-defence
(1) Whoever commits an act in self-defence does not act unlawfully.
(2) ‘Self-defence’ means any defensive action which is necessary to avert a present unlawful attack on oneself or another.
Does the castle doctrine apply in getting them to leave?
Short Answer: Reasonable force may be used to remove the guest as they have become a trespasser - killing them in the circumstances described would be unreasonable and excessive force.
Long Answer: The Texas Castle Doctrine, referred to as Deadly Force In Defence Of Person at section 9.32 of the ...