Complex sentence: After his death, his successor Muhammad bin Tughlaq, moved back to Delhi and left Tughluqabad to gradually crumble and decay, although the tomb of Ghiyas remains there in a very well preserved condition.
Split into simpler sentences: After his death, his successor Muhammad bin Tughlaq, moved back to Delhi and left Tughluqabad to gradually crumble and decay. Although the Tughluqabad fortress is now in ruins, the tomb of Ghiyas remains there in a very well preserved condition.