However, Berserkers (like Macemen, the unit on which they're based) appear at the precise moment in the game when city defenses are at their toughest. Walls and a castle give cities big defense bonuses in the medieval era (made worse if my enemy has built Chichen Itza). But the real kicker is that there's absolutely no way to remove those high city defenses from a water unit in that era. That ability doesn't come along until Chemistry and Frigates.
Oh goody, I get to give advice that is actually accurate and no one else has given yet. A rare occurance to say the very least. Everyone that has read my builderesque posts urging Wonders upon Wonders has figured out I am a builder by nature. That being said my first wars tend to be in the Medieval period. And when it comes to war I usually don't have enough units, especially since I tend to play AggAi with the teams hordes of the Ai. So i have to be clever rather than strong. My technique of winning wars with wimpy armies is complicated but usually very effective.
First off, frigates shmigates, we doan nee no steenking frigates. We need SPIES and eps. Being a builder I love the Great Wall for 2 reasons. A. no barb issues so i can build shiny things rather than troops for barbs. And the early Great Sneak. Settle the guy. Over the course of a game you get way more eps than an infiltration. Plus a little added science for that city. If you had settled the Great Spy in the Gilgamesh game you would have had him for 320 turns. That is 3800

PLUS 960 raw

before building modifiers. And how many

did you lose when you vanquished Washington?
Without a Great spy, you can slowly collect the

in two ways. Adjusting the slider either all the way for a quick boost against your opponent or 10% while you build up your army. Secondly you can assign a spy specialist in each city. You get more bang for your buck from the specialist since you get a little science as well. Just don't do it in a GP farm or you pollute the pool. Though I really like settled great spies so I like when they appear.
the goal is not to collect a big gob of points to steal techs, but to have enough for a city revolt. Preferably enough for 2-3 tries in case the first one fails. I try and put three spies in a city a couple turns before the attack.
Now that we know how to get the defenses down to zero, we need to deal with the stacks of troops the Ai has been building since the dawn of time. I build a nice fat stack of troop eaters. How many of each depends on what your enemy has his army composed of. The basic units are Drill x-bows, formation pikes (which you can get from a barracks and either theology or vassalage when playing an agg leader.), hilldefense longbows and a handfull of suicide cats and flanking2 HAs. Send this SoD (stack of Defense) into his empire on the opposite side of where you want to attack. This draws his troops away from where you are attacking. Now fortfify yourself for the coming waves. A hill is dandy, near a river helps too. or if he has forest or jungle left that is nice. You are probably attacking his nicer cities so your SoD is in the region of his undeveloped ones with jungle or forests still intact. If the AI doesn't take the bait, then proceed to pilliage him into oblivion until he responds. The AI will throw tons of troops against this stack. The stack usually dies, but it takes down a lot more enemy troops. And macemen cost more than x-bows, knights cost a lot more than pikes. The bulk of this stack should be pikes and x-bows since the AggAi favors offensive troops like knights and maces. Just run around pillaging while making your way towards the true attack point. Stopping to heal on hills from time to time once the attacks slow down. Anything that makes it to the city you attack alive will be a heavily promoted beast.
Meanwhile, over in attackpoint B, you use a spy to lower defenses, throw a few cats at the city then assault with beserkers. While they heal, send the galleys back for fresh troops and cats. And send your spies onto a new city when the galleys return with fresh troops and extra defenders, load them up and move down to the next coastal city. I usually research optics while I am building my zerkers so I can have a few caravels helping to protect my galleys. They are stronger than triremes and are useful for inserting spies into a city if you don't have open borders with your victim. It is always nice to get the 50% discount for a stationary spy rather than offloading one and waiting 5 turns while they whip defenders. If you don't have caravels, then offload him at the edge away from your attack point when you DoW then run to where you want to attack.