How to break through the Gate of Oblivion on Jebus in week 2?

Question.

If players have already chosen their starting towns. Let's say Castle, Necropolis, and Dungeon are left. Can the first player choose Castle? Or can the second player also choose Castle? Or is it not allowed to play with the same starting towns?

It's not allowed. The rules describe how the draft works. The first player choosing selects one of the three remaining towns, and the second player chooses from the two remaining and a color.
P.S. It's not worth using that game against Gomunguls as an example, because he was very unlucky with the map; it led him directly to one of the towns in the desert, and that was the end of it. Even with his initial setup, it was impossible for him to win.