immortals! they are pretty good (they are not the same unit as Civ3's, but then again this is a totally different game)
seeing how hard it can be to take a city, and even if its taken, you have to build up its culture, early city-grabbing is a good idea. the conquerd cities are young (lower culture/resistance),every one has archers (thats where the immortals' bonuses kick in), and the immortals can be mass produced fast.
but dont wait too long, the spearmen can devour you.
SO, the early wars (i.e. super ancient expansions)with the Persians are good for many reasons
one being the not-fully-developed enemies(military/culture)
another one being the quick culture build (creative trait!) ability. this mean cities will oftern use full land potential, since the enemy's culture isnt strong enough to create one tile cities!!
p.s. so far i have utilized Cyrus mostly as a peacefull builder, with small wars here and there.