I'm going to give yo one point of advice which, once i understood, bumped me from noble to monarch: specialize your cities. It does not matter if you build the 1.5 workers per city if they just randomly put down improvements. If a city has three fish and a pig and lots of riverside tiles, it would be best as a GP farm (A city that runs lots of specialists to generate great people, which are usually scientists).
If a city has lots of hills, make it a production city. Build only production multipliers and buildings that provide some benefit to units produced here. If you are going for a conquest victory, you will probably want more of these than normal.
The last type of city should probably be the one you will build most often. This is a commerce city and will build only commerce multipliers (That is, buildings that multiply

or

) These should work lots and lots of cottages (There is another strategy for research which involves running lots of scientist specialists but i don't know much about it. Maybe someone else could enlighten us both)
One thing you will need in every city is at least one food source. Obviously more in Gp farms but also more in production cities. This is because sources strong in

are often weak in

.
Now, back on topic. When you are building your Stack Of Doom, be sure to include the following:
Siege
Enough units to take the cities. (Usually city-raider units)
Siege
Enough units to defend the cities you take (Archers or longbows or city garrison rifles, ect.)
Siege
Defensive units to protect your stack (spearmen, shock-promoted axemen, any unit that gets a
defesive bonus against a specific unit or type of units. Chariots are no good because they have to attack to get their bonus)
Siege
A medic unit (So that your injured units heal faster. A GG can promote a unit to medic three, so this is usually what you do with your first GG, usually on a chariot)
And finally, MORE SIEGE!
The emphasis on siege is because on their own against a city, even city raider three units have a tough time against defensive units such as archers. And siege units can also bombard a city's defenses down to zero, so it is just unit vs. unit. You need lots of it because you will keep losing it.
Lastly, if your game gets to the modern era, nukes are beautiful. Two or three will completely eliminate ALL units in a tile. Nice way to destroy a much more powerful enemy's Stack Of Doom.