Slitherine

So i've been playing Spartan for about a year as the Lacedomians and for some reason no matter WHAT i do, i cannot ward off the Persians. Sometimes i get sick of them so i march over with my 16 unit elite hoplite spartiates and conquer half there empire and then in 10 turns it says "Invasion!" and theres another billion guys coming down in 10 ships and i have no way of prtecting the cities just took.
Any help with this please?!?!?

What is the difficulty setting you're playing with? Is it the easiest one?

yes it is

Catch them up early, before they expand into Asia minor. Use cavalry to charge their undefended archers, your infantry is superior (expect for some swordmen, they were tough)

Take a city worth taking, like Rhodes. Build a light infantry barracks and a skirmisher barracks. When Persian ships appear, deploy light infantry and skirmishers in ships to attack the Persians at sea.