Before attacking a castle, the first thing to do is to plan the execution.
To do so, you need to know what is facing you. There are 2 ways of knowing what you have to face.
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Out of the two methods, spying is my favorite because if your first spy attempt succeeds, your target will not even know you spied his castle. Spying also has the GREAT advantage of showing the wall level. I usually send my copper in batches of 101. When a player spawn, the birth castle contains 100 copper. Most players don't know how to get copper so 101 would be enough. If my spy gets caught, I evaluate how much copper I think I need to have a successful report on the second time. Getting caught multiple time can wake doubts in your enemy mind. Point your interest to the farm level because a low level farm won't allow your target to have a strong army at home. Point your interest to the fortification level because the fort favors the defenders with a defense bonus. A lvl 20 wall will double a defender's troops efficiency. Say you have a lvl 20 wall with 200 spears is like having 400 spears. Also look at the troops present in the castle. Spears, Xbows and AH are the units that will deal you the most damage. Swords are decent in defense and archers and lancers are especially bad at defending. The troop types present as reserve in the castle will determine the level of challenge you will be facing.
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The 101 method is a nice way to get your information on the first attempt. It is called 101 because you send 101 troops. As soon as you have more than 100 troops, the battle will last two rounds. When you have less than 100 troops, if the opponent is stronger, the battle will end showing a report with lost troops. You want your battle to last two rounds to get a report with the troops present defending. I usually send 107 or even 111 troops. There are defensive tricks to cancel a 101 attack with 101 troops. A 101 is not the best way to gather real information because of the defender sees your attack incoming, he can hide troops so you don't really know what is in front of you.
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If you are taking part in a op, you must respect the instructions given by the commanders. Often, we don't want the enemy to know we have interest in his castle before your attack arrive.
If you are on your own, you still want to give the least notice possible before your attack.
My recommendation for every attack on a medium strong player: send your spy or 101 only a few hours or minutes before you attack.