This is one of the most popular strategy used these days in Age of Empires and is based upon the Knight Rush.
1.Civs favorable for fast castle strategy:-The cavalry civilizations are favorable with huns being hot fav.Particularly those civs which have an advantage of research are better.Examples are franks;their knights are 20% stronger and they have free farm research.Huns save wood as they dont make houses,vikings have free wheelbarrow ,so faster vils,persians have faster work rate etc.
2.Timing:-The ideal castle age timing is around 16 mins.I have seen ppl doing castle age as early as 13 mins but its not of any use.
3.The Fast Castle Strategy:-
HCCCCbuild 2 houses(1 house with 1 vil and the other with 2 vils)
Scout:-Explore with scout in circles.As soon as you find 4 sheep,bring 1 to TC for eating,keep others for exploring.
>>Keep clicking vils unless you have 6 on sheep and 3 on wood.
Click Loom:-While loom is being done build a lumber camp with the 3 vils on wood.
10th vil on boar,build a house near boar and then lure it.Make sure that the 6 vils having sheep finish it before they hunt the boar.
Click vils on bush and build a mill near it.
16th vil:-Goes to lure the second boar and builds a house.
17th vil goes for wood
vils upto 20 go for sheep
21st,22nd,23rd are farmers.
24th and 25th go to mine gold.
Click Feud:-While going feud you can either go for hunting with 4 vils or go for farming according to your civ.Make sure you have 300 wood as soon as you reach feud.
Feudal Age:-Once in feudal age click 5 vils on gold.
With 2 vils build a smithy and a market.
As soon as you have 30/30 you will be in a position to click castle.
While advancing do the lumber camp research and build one barrack and 2 stables.
Take care that you have enough food and gold so that you can take out atleast 4 knights(2 from each stable).
Castle age:-Once in castle click on 4 knights and build a TC near your main gold mine or your lumber camp.
Knights:-They are very costly,so take care with their attack,use them to raid the enemy's base,dont go near to his TC and keep
away from spearmans.
4.Precautions to be taken:-As your are going fast castle you are vulnerable to flush(Feudal Rush) and trush(Tower rush).Use the house bug to detect them and build walls wherever necessary.
5.Effective use:-Dont let your knights idle around.If you are successful in confining your opponent near his TC,then make siege and destroy his TC with onager or ram.
6.Aztecs and Mayans:-These 2 civs dont have knights.Explore the option of Eagle warrior.They are much cheaper so can be made more in number.Although they are weaker than knights they are very effective in raiding.For these two civs lay more emphasis on gold than food.And the important research is the first smithy research.For more info read The Eagle Warrior.