It is archaeologically difficult to distinguish Diocletian's fortifications from those of his successors and predecessors. The Sunnis recognize the first four caliphs as Muhammad's rightful successors. The trustees themselves are responsible for choosing their successors. Once in office he revived the practice of presidents designating their successors. His three immediate successors were assassinated as well. The board is self-perpetuating with trustees responsible for choosing their successors. Although superseded by their stone successors, timber and earthwork castles were by no means useless. They were the successors of the Hittite Kingdom. The Khalsas also agreed to never interact with those who followed rivals or their successors. He exerted considerable influence on his successors. The successors of Obelerio inherited a united Venice. He saw his tetrarchic system fail, torn by the selfish ambitions of his successors. None of his successors have used it. The region remained under the control of his successors Rudak and Bhima. Sparta refused to see Lysander or his successors dominate.