Generally speaking when attacking a city you want to have at least 3x as many archers as that city has archer towers. PLUS you need to account for the archers and other troops if the gates are open.
For example. The enemy player has 1,000 archer towers, 1500 traps, 500 archers, and 2000 warriors.
That works out to be about 5,000 total defenses.
I’d suggest sending in at least 3x that amount. I’d prefer to send in at least 7500 archers and 7500 warriors. That should win pretty easily. 10,000 archers and 5000 warriors would probably win even easier.
There is an advanced Evony strategy that it is much better to win big rather than win a close fight. You might want to make a habit of sending 4x as many troops if you can.
You also might want to find out if the gates are opening before sending in your army, because you’d need a lot less troops!