Complex sentence: It was owned by KaibaCorp and was a Military Base before it was shut down and destroyed by Seto Kaiba and built the Duel Tower on the ruins of the base to show that the company had shifted from military to gaming.
Split into simpler sentences: It was owned by KaibaCorp and was a Military Base before it was shut down and destroyed by Seto Kaiba. He then built the Duel Tower on the ruins of the base to show that the company had shifted from military to gaming.