King Aston is the ruler of a the rich merchant kingdom of Asturia. However, the country is not militarily powerful, as is shown in episode 6 City of Intrigue, when the King rebukes Allen for possibly instigating a war that Asturia does not have the military might to win.
King Aston initially appears to be greedy — during episodes 6 and 7, one of his advisors suggests that upon seeing Escaflowne's power in battle that he could obtain a large amount of gold by selling it.
The fact that this would betray Fanelia's own King seemed to matter little and highlights the fact that their so-called alliance with Zaibach is only really for show.
Later, however, it becomes clearer that King Aston is afraid of Zaibach, and feels that he must capitulate to them or see his country destroyed.