Complex sentence: During these years he continued the traditional pattern of Ottoman architecture; but he gradually began exploring other possibilities, because, during his military career, he had had the opportunity to study the architectural monuments in the conquered cities of Europe and the Middle East.
Split into simpler sentences: During these years he continued the traditional pattern of Ottoman architecture. But he gradually began exploring other possibilities, because, during his military career, he had the opportunity to study the architectural monuments in the conquered cities of Europe and the Middle East.