You can use "en" when using "at" as inside a building.
¿Por qué estás en mi casa?
(the subject "tu" (you) is hidden in this phrase as it can be understood from context).
The preposition "a" implies direction,
me voy a casa -> I'm going home
The preposition "de" implies relation or posession.
salgo de casa -> I'm leaving home
la casa de mi padre -> my father's home
el gobierno de Inglaterra -> England's government
Also, the Spanish equivalent of "What on earth are..." would be
"¿Qué diablos son..."
(wich roughly translates as "Which kind of devils are..." :D)