Notes

You can do the following with any civilisation that has knights. But Huns, Teutons, Mongols, Franks, Persians and Teutons are most viable. Also cav. archers is only viable with Huns.

In a fast castle strategy we wish to reach the castle age as fast as possible without leaving the town centre idle at any point. Each second that the TC stays idle at the beginning of the game may have an impact of 10-15 resources by the time you reach castle. So if your TC stayed idle for a minute, you may be 600 resources behind by then(and that translates to 5 less knights in your army)

The following guide is for land maps. I will give detailed steps on how to do a fast castle. This is mostly recommended if you are in the pocket position of your team in a 3v3 or a 4v4. I will also cover how to counter any early aggression by your opponents and a bit of how to continue with a fast castle aggressively. I will put square brackets for unfavourable situations that happen sometimes.

Keep training villagers at the TC. Build a house with 2 villagers and another with 1 at the start of the game. This will ensure that the TC is not idle when it completes its first villager. The scout must try to find the nearest set of 4 sheep. [If I don't find them by the time houses are done, I send a villager to help in scouting and all others gather from a nearby strangler tree meanwhile.]
Once you have a sheep under your TC, make all villagers shepherd. I make the first 6 villagers shepherds. [If I had started shepherding after the one minute point (because of late sheep find) I make 7 shepherds to compensate for that.] With Britons you will need only 5. This ensures that villagers keep popping out of the TC.
With regards to shepherding, ensure that you only have one dead sheep at a time. When 6 shepherds gather from 1 sheep, they collect only 89 food (11 rots away). If 3 shepherds each were gathering from 2 sheep, they collect 80 from each. I ensure this by keeping only 1 sheep near current dead sheep. Other sheep should stay outside. You may even scout with them. But keep sending them back to ensure that 1 alive sheep is under the TC. Don't scout with sheep after 2-3 minutes in game time. Also note that sometimes you have force drop food when you don't have enough to train the next villager
Then I send my next villager to good nearby wood line and make a lumber camp touching the trees. You will need 4 lumberjacks. Meanwhile keep scouting to find your 2 boar, other 4 sheep and your gold and stone mines and deer patch. After that proceed to find the enemy.
I send the 11th villager to lure a boar. [If you are not good enough at this, then do loom. You can usually avoid this if the boar is not too far away.] Ensure that the boar dies under your TC. You will usually need to do force drops to keep villager production.
All my shepherds would be collecting food from the boar now. I tend to follow only one meat rots at one time policy. Have 7-8 villagers on boar and send new villagers to the berry patch. The first one should also build the mill. Meanwhile build a house with a lumberjack (or the first forager). I tend to have 3-4 villagers on berries.
By the time you have ~180 food left on your boar, send a villager to lure the other boar. Build 3 farms using the injured villagers. Build another house. New villagers are sent to collect food from boar or berries. At 19 villager point you must have about 4 lumberjacks, 3 farmers, 4 foragers and 8 boar hunters. Send the next 5 villagers to lumber. Build another lumber camp. Distribute the lumberjacks between the two camps evenly.
When the boar is depleted send them to gather food from remaining sheep. You may send 4 of them to your deer patch. You can either lure them with your scout or build a mill there. However I recommend using the scout to scout your enemy. You must find out whether he is planning a fast feudal attack. This is usually the case if they reach Feudal age before 11 minutes. Also if you see that enemy has a barracks at 7 minutes, he may be planning a dark age rush. I will discuss in the end how to deal with such threats.
Send the next 3 villagers (25-27) to gold mine. Then research loom if you haven't already and click Feudal age button. We should have 3 gold miners, 9 lumberjack and 15 food collectors. When advancing to the next age, build a barracks as well.
Once you reach Feudal age, queue 2 villagers at the TC. Build a blacksmith and a stable/archery range with 2 and 3 villagers respectively. By the time these 2 villagers are trained you must be ready to advance to the castle age. During the castle age advancement research 2-bit axe and horse collar.
The 26 villagers, loom and age advancements take 16:05 time. Good players should reach castle by 16:30 considering game lag.

Depending on whether you are doing knights or cav. archers/crossbowman, the strategies diverge. Note we need about 6 farmers and 4 lumberjacks to support 1 TC.

Knights (Teutons, Franks, Mongols, Persians)

To keep training knights from 1 stable we need 6 farmers and 7 gold miners. During the advance time build another stable. Research bloodlines in one of them. Make farms when berries/deer run out. If you don't have enough wood, these villagers can temporarily chop wood. Make the farms under your TC preferably. Your 29 villagers should be like
8 lumberjacks, 14 food gatherers, 6 gold miners and 1 builder.

Once you reach the castle age, research wheelbarrow. Train 1 knight in both stables. Add newer villagers to gold. When you can afford, research gold mining and scale barding armour followed by chain barding armour. By 37 villagers, you should have 18 farmers, 13 gold miners and 6 lumberjacks. This is a good point to get hand cart. Then you need to decide whether to support a 3rd stable or a 2nd TC. Try to minimize both stable as well as TC idle time. And don't forget to attack with those knights.

Cavalry Archers (Huns)

To keep training CA from a Hun archery range, we need 2.5 lumberjacks and 5 gold miners. During the advance time build another archery range and research fletching. Redistribute berry/deer villagers to wood/gold/farm. Your 29 villagers should be like 15 lumberjacks, 7 farmers, 6 gold miners and 1 builder. When you reach castle age, do wheelbarrow and queue CA in the 2 archery ranges. Research bodkin arrow as soon as possible. The blacksmith researches are more important here. New villagers go to gold. Add the 3rd archery range. When you are able to afford, research gold mining, bow saw and ballistics (with university). By 37 villagers you will need 15 wood cutters, 13 gold miners and 9 farmers. You will need to build mangonels to destroy enemy buildings with this army however. Get archer armour and bloodlines if fighting archers.

Crossbowmen (Mayans, Britons)

Mayans need 2 lumberjacks and 4 gold miners per archery range. Britons need 3 and 6. With Mayans 1 TC and 4 or 5 archery ranges is preferable. With Britons 2 TC and 2 or 3 archery ranges. The buildup is very similar to CA above. However you can start training your army much sooner.

Countering early enemy aggression

Use your scout to figure out what the enemy is trying to do. Let us deal each type of aggression case by case. Note that it is always safe to have a small stockpile of wood as it allows you to immediately build the appropriate military building if you detect any threat.

Drush: If you see enemy building an early barracks, it indicates a drush (an early dark age attack with 3 or 4 militia and the scout). Immediately research loom. If it is possible, try to build a palisade wall to secure your base. You may forgo hunting deer as they are the most vulnerable targets for a drush. Instead build 2 more farms. You can also counter it by training your militia if you scout it before 7 minute mark. In this case you will be advancing to feudal age with one more villager to cover their cost.

Scout Rush: If the enemy advances to Feudal age quickly and starts building a stable, it indicates a scout rush. If detected, this is the easy to counter. Train 2 or 3 spearmen from your barracks. Place 1 in defensive stance near each villager group. If they attack, try to chase the scouts away using that spearman and nearby villagers.

Tower Rush: If the enemy seems to be mining stone in the early game, it indicates he is trying a tower rush. Be vigilant with your scout to see if villagers are moving towards you. If you counter a tower rush, you will most probably win as your enemy would have spent too much resources in a failed attack. So it is often better to stay back in Feudal and ruin your enemy's plans. Try to prevent any tower from going up. If any do get constructed, you can limit their spread by constructing your own tower(s). Try to secure important resources like gold/lumber camp using your tower(s). You can bring down isolated enemy towers using your villagers. Villagers have an attack bonus against towers. You can also try attacking his base using scouts/archers. It is likely to be undefended.

Man-at-Arms: This is the easiest to handle early aggression. Just train some archers and research fletching. Archers have range and are faster than man-at-arms.

Archer Rush: If you see 1 or 2 quick archery ranges, it indicates an archer rush. This is the hardest to handle if you want to fast castle. Building walls can buy you enough time if you can complete them. Towers can protect areas from archer harassment. But with starting stone you can only build 1 tower. It is possible to fight cost effectively by training skirmishers. But note that you will need good numbers. This can delay your fast Castle plans massively. Also skirmishers are vulnerable if enemy mixes some scouts in his army. Even if you do manage to reach Castle age, you aren't immediately ready to fight back with knights or cavalry archers. If the enemy manages to accumulate about 20 archers, they will kill fresh knights in 2 volleys. The best thing you can do is to train a mangonel and hope it gets a lucky shot at your enemy.

Also keep scouting to see if your enemy is changing his plans. Keep looking at the minimap at regular intervals.