In my alternate timeline, the original Atlantean civilization perished around 1,500 BCE. Around 1000 BCE, humans once again find Atlantis, this time with 10,000 Greek outcasts fleeing from persecution from their homeland, and over the course of a 1000 years, founded a empire of over 10 million people and technology far more advanced than their neighbors, equivalent to the late medieval age(1400s, gunpowder). But around the year 100 AD, the Han Dynasty of China, the Persian Parthian Empire, and the Roman Empire(ignore the impossibilities) unite in order to destroy their formidable neighbor. Now, this alliance is formidable, with a combined force of over a million and a half personnel, and 150 million people-a full 15 times the population and personnel. The alliance is preparing a huge fleet to transport 750,000 troops to Atlantis over the course of a decade, and the invasion will commence in 5 years.
Assuming that the Neo-Atlanteans have essentially limitless resources, and can call upon 2.5 million people to work for the government, what is the best way for them to prepare a defense and beat the alliance?
Tactical & Strategic Information: The organization of the Neo-Atlantean Empire has a central city with 3 million people, the two rings each hold 3.5 million people, and there are 4 urban centers on every ring, each holding about 250,000 people. The Neo-Atlanteans have 100,000 personnel, 50% infantry, 25% calvary, and 25% ranged(arquebus). The invasion force is 90% infantry, 5% calvary, and 5% archers. The invasion force brings ballistas, battering rams, and other siege equipment. The Neo-Atlanteans have advanced trebuchets, catapults, cannons, and guns. They have a system of roads and a series of artificial bridges that crosses the inland seas.
Geography: No mountains, mostly forest or grasslands, 1 km deep inland seas. 500km radius of entire island, 25km wide inland seas, 75km radius central island, and 150 km thick rings.
Goal: Make the alliance invasion retreat, kill all of the invasion force, prevent the majority of the population from dying or being enslaved.