"A lot of animals were at my party."
Translation:Muchos animales estuvieron en mi fiesta.
The preposition 'a' usually indicates motion, manner, or to express exact time. Por ejemplo:
Fui a la pelicula = I went to the movie.
Fui a pie = I went by/on foot.
Fui a las tres = I went at three.
'en' is often used to indicate location (like Duo's sentence) or a time period (like 'summer', or 'in one hour')
Estaba en el cine = I was at the movie theater.
Fui en el verano = I went in the summer
Prepositions are tough and I'm still learning as well so keep in mind this is by no means a complete list of uses and sometimes they (prepositions) seem somewhat arbitrarily assigned. I always recommend SpanishDict as a good source if you want to learn more.
It seems to me that the imperfecto estaban would also be correct. Is there a native speaker or advanced student who could clarify? Gracias de antemano.