Learn Polish: Mój rycerz rzucił tarczę i miecz poza mur. - My knight threw the shield and the sword beyond the wall.

The preposition poza takes the accusative case when it describes motion or direction toward a place (answering the question 'where to?'). Because the knight threw the items to a place beyond the wall, we use the accusative mur. If the items were just sitting outside the wall, with no motion involved, we would use the instrumental case: poza murem.

Why does the shield change to tarczę while the sword stays as miecz? Both are being thrown!

Both words are in the accusative case because they are the direct objects being acted upon by the verb. However, tarcza is a feminine noun, and feminine nouns ending in -a change to -ę in the accusative. Miecz is a masculine inanimate noun, and for these, the accusative form looks exactly the same as the basic dictionary form.

Could I use the verb rzucał instead of rzucił here?

You would only use rzucał (the imperfective form) if you wanted to say he was in the middle of throwing them or threw them repeatedly. Because rzucił is perfective, it shows that the action was a single, completed event: he threw them once, and it is finished.