The Castle Entrance is unlocked when you complete The Forest. It contains Knights which have 150 HP each. The easiest ways to get through are to get A Magical Sponge and use "squeeze" to go under the guards, or to use the Rocket Boots to jump over the castle wall.
After successfully completing this quest, you can move on to The Castle.
A new Knight will spawn whenever the rightmost Knight is either past the player or has <40% HP. This results in a swarm of Knights as you get closer and closer to the end of the stage.
With the Boots of Introspection stand just past the entrance and kill enemies for 2000 candies per kill.
Or you can use the Boots of Introspection to remain where you spawned and use the Black demons spell acquired via the Black magic grimoire to greatly increase the amount of candies earned. Using this spell is most effective when standing to the far left. Combine with Candy Merchant's Hat to increase effectiveness even more.
Another tip is with the pink gloves and a powerful weapon, this can be the fastest and most effective way of earning candies.
There is actually a way to get 10000 to 15000 candies per second here; this player had scythe, merchant hat, enchanted knight body armour, pink enchanted gloves, boots of introspection, 20% attack boost, 20% health boost, 1200 max HP, eaten >391096 candies. The player jumped with the boots of introspection and landed on the ground in the spot on the map so that their character was against the end boundary (both arms still showing, but the next text space is the boundary, so no space between the player's arm and the end) An even better location is that of a player who jumped with the boots of introspection and landed on a knight so that they were nine rows above the line that also has the bottom of the castle wall, against the end boundary of the quest with one arm hidden, and because the knights generate there, they are almost instantly killed. This glitch nets in with these stats about 15000 candies/second. At this rate, one would have 1.296 billion candies in 24 hours. Sometimes there are periods of higher/lower candy gain, but just rake in whatever comes; even 1.0 billion is still an enormous quantity. Just remember to finish the quest by escaping the glitch spot (teleporting works best) or your candies will not be saved.
Illustrated: (Any of these --> ¶ will be used to represent the end boundary of the level)
Ground level tactic (medium-hard): (the horse's head may flash in and out of existence, that's fine) ¶ \o/¶ ¶ High level tactic (very hard) 12 ¶ 11 ¶ 10 ¶ 9 \o¶ 8 ¶ 7 ¶ 6 ¶ 5 ¶ 4 ¶ 3 ¶ 2 ¶ 1 (far to the left is the bottom of the castle wall) ¶
Different locations yield different rates; surely there is one that may be better than this.