Plan for the plot is for your chars to have come to Ylisse from their own time period and place in the FE world. This will have happened before the actual plot begins, which will be in the Ylisstol Courtyard as described in the OP. Not everyone will be escorted into the courtyard in exactly that manner, but somehow, they would have traveled through the Outrealms to this time, whether with the Knights of Ylisse or simply by their own curiosity leading them to step through an Outrealm Gate.