Within 30 days after the last publication of summons, any person interested may appear and answer the complaint. The answer shall set forth the facts relied upon to show the invalidity of the district. If no answer is filed within the time allowed the court shall render judgment as prayed for in the complaint. If an answer is filed the court shall proceed as in other civil cases. The proceeding is in rem and the judgment rendered is conclusive against all persons who could have appeared in the action and against the State.