Simplify the following text for a Grade 10 native English-speaking person.
Original: He held the imperial seal and could convene all other viziers to attend to affairs of the state ; the viziers in conference were called `` KubbealtÄ viziers '' in reference to their meeting place , the KubbealtÄ ( ` under the dome ' ) in TopkapÄ Palace .
Simplified: He held the imperial seal and he could gather all other viziers to attend meetings regarding state affairs .