Another thing you should look at is whether the castle has walls.  If a level 40 castle has full level 5 walls, there's a good chance it's a trap. 

Another thing you should look at is whether the castle has walls.  If a level 40 castle has full level 5 walls, there's a good chance it's a trap. 

 True!

Also it is a good idea to spy first rather than sending blind raid.. Spying and selective raiding will help you to get most out of the castles that you raid..

Also it can help you make a list of castles that you can raid regularly for resources incase they have good resources every time you raid. 

you can make a list in your contacts and keep visiting them for resources at regular intervals. you could also calculate the time by when it can give you max resources by checking it over a few days and noting it down on the contacts. this way you will know when will it be best to raid it without spying.

another good idea is to keep an archer in the castle after you raid as a reinforcement to understand if anyone else raids it. This will help you keep an eye on the castle without have to flag it as inactive by seiging it

Good tips, Warrior!