Hi. Love the premise.
I’m in the middle of the forest campaign. Free account.
The game is a chore so far. Everything I do is just mechanically translate comments into code. Trying to charge through the tutorials but they just never end. Running out of steam.
I understand that the game is meant for teaching, but I’m a software engineer and don’t require teaching. When does a challenge starts? What are the paid levels? Are they hard or just different?
Things get interesting around the Desert, and for sure in the Mountain.
We’re in the process of expanding the Forest significantly to help younger players grasp the computer science fundamentals. For our more experienced programmers, I suggest some of the following:
Subscribers get access to more levels, and, additional hero types like the Ranger and Wizard. Rangers have access to long-range weaponry and throwing sidearms, while Wizards have wands/staves and magical tome sidearms allowing them to cast various abilities. They are useful in dueling and repeatable levels.
I recommend trying to get up into the Mountain, that’s when you can start summoning and commanding troops which opens the doors for a bunch of unique play opportunities.
Regarding the main campaign, the first challenging level for me was at the very end of the Mountain map, the Summit’s Gate. If I recall correctly, it is the first level where you are not mechanically translating comments into code nor building up on simple algorithms from previous levels.
Summit’s Gate actually requires real strategy, and is a quite complex 3 parts level with close to no guidance—it surely is a huge spike in difficulty. Back then, it was a real challenge for free-to-play players. I had to go to back and redo repeatable levels to afford better gear. Nowadays there are so many more levels and achievements, you will probably have enough to afford end-game gear by the time you get to Summit’s Gate even in the free-to-play campaign mode.
After Summit’s Gate, you have the whole Glacier map with a similar or even higher difficulty. It is a pretty good pastime. 
Apart from the main campaign, Serg’s comment provides several ways to have fun with CodeCombat even for experienced programmers. I personally enjoy the “Brawl” levels where you have to stay alive for a determined amount of time and they get harder with each victory.
Yeah well… end of Mountain, huh… sorry to say but I cannot make it that far. I’m just exhausted by the game, it’s worse than bugfixging. There should have been a shortcut like in the cave… except then there will be that money problem, so that’s not an option.
As for Serg’s suggestions… while I appreciate him trying to help, they just don’t work do they. I mean, the tasks are: call functions, conditions, and cycles. Any imperative language is similar at this level, more so Python, Lua and JS, which are quite similar to each other. Compare one of these to Haskell, for a counterexample. Top leaderboards require top gear, so that doesn’t work either. Even if they didn’t, it doesn’t push progress to harder levels. Besides, I’m not a kind of person who enjoys aimless optimizations for the sake of the total score Repeatable and arenas don’t work for the same reasons. Arenas are also frustrating because they include heroes who one shot me with triple crossbows and magic when I don’t have access to these items.
I think this game is a lost cause for me, sadly. I’ve found another one with fantasy elements, a whole range of different languages and selectable difficulties. No hack and slash elements, sadly.
So, if Hyppe is still reading, I am curious (as I am sure others are) to know what this other site you have found is.
I have tried getting into the top 5 on several levels but am confused why there seems to be a big disparity between players in damage dealt when solutions seem to be running same code, same hero, looks like even same or similar weapons. I have re-run my solutions on Eagle Eye level, for example, using same code as what nearly all the solutions I can see are using, using same hero, and trying with both high damage and low damage weapons but can’t seem to even get into the top 20. Can someone explain to me the big differences in damage dealt scores?
Well if I’m allowed to say, it’s codingame. There is a mention of it in this forum. The game has a puzzle oriented training mode and a multiplayer mode, which is fair, competitive, and optimization oriented .
@Hyppe why not skip the game and try building levels instead?
Since you already know programming, building on to the game should be a challenge. They have also open sourced the engine and I believe, have actual code bugs still lingering around that need fixing…
I mean if you think of it “slaying the dreaded code bug” might be the greatest challenge of all ? 
( http://gadgetblur.com/another-bug-fix/ ) for a laugh.
more geek humor: (the image)
https://rules.ssw.com.au/management-is-your-client-clear-on-the-definition-of-a-bug