Against phalanx units, and only having weak sword infantry and basic skirmishers, you can expect heavy casualties in frontal attacks during a settlement battle. Phalanx units at their most lethal in these narrow-quarter battles.

You're on the right track though, showering the enemy with missiles. Before you enter the city, the AI often wastes its men by walking them within skirmisher range and in front of the gates. Keep firing at these idiots when they walk past. If missiles are running low, turn off fire-at-will mode, and only tell your skirmishers to throw their javelins when the enemy are walking from left to right so that their shield is on the wrong side to protect them from the missiles. (I'm assuming this is a wooden wall city and not a stone one.)

And then prepare for carnage as per usual I suppose. If you have the time, try and make one breach around the side of the enemy settlement, so that when your main army attacks through the front of the city, you have a couple of units able to sneak in and attack the enemy from behind. Won't work if the enemy keeps some units in reserve at the Town square mind. Cavalry is good for flank/rear attacks though.

You'll really rely on making on the enemy rout when attacking them in the streets. Once they rout and they have their back to you, cavalry will finish them off fast. Routing them is the tricky thing though. You need to break up the phalanx formation. Cavalry charges can work well, but if the initial cavalry charge comes to nothing, you'll take large losses. To break up the phalanx line, you could tell you cavalry or infantry to attack a unit behind the nearest phalanx unit, so that they try to run through it. This may sound suicidal, but your men will try and break through the line of the first phalanx unit to reach their goal, hopefully breaking up the rigid phalanx formation, and forcing the hoplites to use their rubbish swords which makes them easier to kill. Often if you just click on the first phalanx unit to attack, your men get stuck on the spears and struggle to get near enough to inflict damage. See near photo-realistic demonstration below:

Eg:
HHHH -- Hoplites waiting to poke your men, Attack these
HHHH -- Hoplites poking your men
IIIIIII -- Your infantry getting poked.
IIIIIII -- Your infantry waiting to get poked and possibly wetting themselves at the thought.

With Hastati, the battle is easier: have them on fire at will mode and line them up behind one another when you enter the city. Let the Hastati fire at the enemy on their own accord, and each volley should wipe out a large number of the enemy hoplites. A couple of volleys at close range might rout some hoplites too.

Try and hire some merc hoplites for battles against the Greeks. They'll help level the playing field.

And that's all the useless information I can give regarding your situation. Try bringing along some dogs or Scutarii or whatever else you can next time to give yourself more options.