Ah, sorry, I was absent here for most of the day. I haven't seen Amnon's solution yet, so I don't know if his moves are exactly the same as mine, but the "big picture" should be the same -- there are simply no other options. (EDIT: Just watched and it's exactly the same move for move, finally I made level that's truly airtight in the FIRST version!)
This should be an easy level for you guys right?
Not exactly. For a long time Amnon was the only one who responded in this thread, even though the level was downloaded 23 times. That means, at that point, potentially there were 22 others who didn't solve it. Cookie needed a few hints from me in a Discord server, and it was fun watching a live report of his reactions.
Don't feel disappointed if you couldn't do it. It's a pretty hard level, indeed.
Never understand why you can create and solve such complicated levels
Creator's side: I
very rarely post RTW levels nowadays, but when I do, I generally try to push myself to the limit and create the trickiest, hardest and most "out there" puzzle.
Forgive me if I sound pretentious, but as time went on, I found out that this is the kind of puzzles that inspires me both as a designer and as a player. Not just the most difficult one, but the most
elegantly difficult one.
It was said of one chess player (Frank Marshall) that some of his most brilliant moves at first looked like typographical errors. Many of my favorite puzzles, at first, look like design errors. They seem like they can't be solved, so when a solution is finally found, it feels like a bit of a miracle, a magic trick of puzzle design.
Obviously, there are puzzles that are too hard for me to solve, but I have solved some pretty difficult levels. Mark's "Master of the Game" is one of my favorites -- Mark in general is really good at incredibly elegant puzzle design.
Outside of Wonderland, there's stuff like Portal Pro, a mod for Portal 1 by Mevious. It has some levels like that. "Ride" is a stroke of genius that took me over 2 hours and I'll never forget the moment the solution "clicked" in my head. It's a beautiful, creative level. And the less I talk about how embarrassingly long it took me to figure out the central trick to "Sewage", the better.
Whenever possible, I like to create the same feeling with my own puzzles. It's beautiful when "that's a joke, there's no way this can be solvable, right?" slowly, slowly turns into "oh, THAT's how".
With "Security Through Insanity" in particular, I wanted to create a "there's an object missing" feeling, which is why I added that sign in the end. However, there's no missing object, the level is just that clever.

Mind you, I originally wanted to make it part of a much larger and even
more difficult puzzle, where by looking at the layout, you wouldn't even
know that you need an extra sphere outside. The rest of the level would
look perfectly solvable without it, but wouldn't be. However, due to the nature of what I wanted the extra sphere to be used
for I couldn't figure out how to make it actually work, so the plan fell through and I turned this into a separate level.
this is one of the hardest levels here
Amnon, you mean one of the hardest *recent* levels, right? On your end, some of your own levels are pretty tough, then there's dkgiz and LEMARQUIS... and on my end, "Deceptively Deceptive" is a thing.

So there are plenty of harder levels in Wonderland history, but as far as recent levels go, yes, this is a really tough one.