We don't know if there were any other altars, but it's possible that the altar we found was the only one meant for the Bridge. [England's sacrifice had been considered necessary for a reason, he thinks.]

And I'm not sure that's what it means...if the Bridge is ancient, it could just mean that she enjoys sacrifices like the older gods in our world. You're from Earth too, aren't you, Ogino-san? [It's a fair bet given her usage of honorifics, but who really knows these days.]