Learn German: Aufgrund des Befehls hat das Katapult große Steine geworfen. - Because of the order, the catapult threw large stones.

The preposition aufgrund (because of / due to) requires the genitive case. Since der Befehl (the order) is a masculine noun, the definite article changes to des, and we add an -s to the end of the noun. In casual spoken German, you might occasionally hear the dative (aufgrund von dem Befehl), but the genitive is the standard you should use at the B2 level.

Why does the adjective in große Steine end in -e rather than -en?

This is an example of strong adjective declension. Because there is no article before Steine (such as die or keine), the adjective itself has to do the heavy lifting to show the case and gender. Since Steine is plural and in the accusative case (it is the direct object being thrown), the adjective takes the strong ending -e. If a definite article were present, it would trigger the weak ending: die großen Steine.

What are the core forms of the verb that gives us geworfen?

geworfen is the past participle (Partizip II) of the strong verb werfen (to throw). Because it's irregular, its vowel shifts across its forms: werfen (infinitive), wirft (he/she/it throws), warf (simple past), and hat geworfen (present perfect).