I was annoyed at first about this issue too, but some thought and practice fixed that. I remembered reading somewhere that all else being equal, defenders have about a 10:1 advantage over attackers in a siege. That's where you start at. Gotta work from there or you'll throw 10x your enemy's soldiers into the blender.
Some siegecraft and misc attacking strats/tactics that I've developed (keep in mind that we're all still learning this game so have a grain of salt handy):
Strategy:
-Time your unit production. Your goal should be to spend as few turns as possible to create your entire army. Using only part of your civ to produce a steady stream of units really really hurts you in the long run. Use every feasible city to create your mounts, then your melee, then your defense units, then your siege units. Ideally these come out sorta as waves.
-When making units, always
always have your civics set to vassalage and theocracy if you have those available. Period. No exceptions. With a barracks, that puts your units at I think 8/10 XP right off the bat. Huge.
-If you don't have roads to the enemy border, build'em. Again this really isn't an option if you're planning on conquest. Roads aren't absolutely necessary for raiding, but that's a different subject.
-Oh and while I'm thinking about it, religion is beautiful. Think of missionaries as intelligence and cash. Any city you've preached the word to gives you vision. Great for ambushing and counter-ambushing. So plan ahead and spread the good word brother.
-Bear in mind that strategy from history is valid here. Starvation was/is a key factor in siegecraft. Use it.
-Be sure to keep your units upgraded. It's cheaper and faster, and produces higher quality units. If necessary, put your research down to 0% for a turn or two and upgrade everything when the new tech is available. I've had my starting warrior survive for a full game and end up with over 20 XP.
Tactics:
-I go mounted/raid units first because I like to "shock and awe" like it's 1999. I place them as close to the enemy boundaries as possible, spread as widely as possible. As soon as they're in place, I blitz. My goal here is to generally raise as much hell as possible. First priority is units. I steal as many workers as I can quickly reach, and kill any military units that are in transit. I don't f*ck with spearmen or elephants. They'll eat me. Second priority is food and special resources
that aren't adjacent to an enemy city. I raze them to the ground. It's a decent bit of cash and it's going to nicely set up my next phase. Then I get the hell out of dodge. I heal when I'm back on friendly ground.
-The melee wave should be out now. Axes, spears, and swords, oh my! This is where those roads come in handy because melees are slowasses. This is my next step of (pre)siege. My goal here is to park on the tiles next to a city. Same priorities as before, only I don't raze. I just fortify my guys on top of farms, mines (especially good since you get hill defense), special tiles, etc. I'm masterfully baiting the enemy here. He has to choose between coming out to meet me and losing his defense bonuses, or starving and not producing anything. Even if he kills my guy, I win because I can then kill
his guy next turn. That's one less unit in his city, and I still control the tile.
Oh and another high priority is chokepoints. Spears and axes are great choke defenders. If I'm really concerned, I park a spear/axe combo in a chokepoint and build a fort there. I really hate having my own tactics used against me, and this is a good counter to raid units.
-My ranged units and freshly healed raiders should be en route now. I use them to harden injured or valuable parts of my army. This is where I finish surrounding a city. Note on longbowmen: I almost always put 2 levels of city defense first as that's what I'm going to do with them 9/10 times.
-Finally, my siege units arrive. I level collateral damage first and always. Heh, I've never had one survive long enough to get anything else. First I bombard the city until its defense % is lower than 5. Then I sacrifice siege units until my combat odds are a sure bet. Then I conquer the city, permanently parking longbows there and temporarily parking injured units there. I plan for a culture bomb if possible.
-If you managed to read this far, have a cookie on the house.
Edits
Good christ there were a billion posts while I wrote this book.
Yes, siege units are expendable. This is historically accurate.
Oh and Vizzini here you go:
Gen. Nathan Bedford Forrest said:
War is a matter of getting there the firstest with the mostest.
I love that quote.