I think the translation of the exercise in §99 II.2 isn't correct. I'm trying to reconcile the phrase "in castra Romana".
English: My son Sextus is carrying his booty to the Roman camp
Key: Filius meus Sextus praedam suam in castra Romana portat.
Meos: Filius meus Sextus praedam suam ad castros Romanos portat.
Here's how I broke this down:
in castra "in (on) the camp" ?
§53 gives the adjective in that has the ablative ...