This is the 14409th most frequent Turkish word.
orduya
"Orduya" means "to the army" in English.
In this context, 'orduya' indicates the dative case, meaning 'to the army', specifying the destination of joining.
Orduya katılmaya karar verdi.
He decided to join the army.
Here, 'orduya' again shows the dative case, used to describe the entity that the enemies are approaching ('to the army').
Düşmanlar orduya yaklaşmaktaydı.
The enemies were approaching the army.
In this sentence, 'orduya' is used in the dative case to indicate the target or entity to which the new units joined ('to the army').