This is a very frustating level (or it is bugged at the time of my comment).

Even if there is currently no bug preventing the level from being completed, it is at bare minimum, very disrespectful of the player's time. Having to arse about with the workshop economy while you try to work out the one and only specific solution the designer seems to have created to beat the level is just tedious busy work. That's not difficulty, it's just tiresome clicking.

With no bridge and no CTA, there is no way to attack any of the heroes on the other side of the lava; except by lightning (and that just straight doesn't work against multiple knights, and, again, would require a stupid amount of time and tedium with the workshop to fund.) I've mined every bit I can get to - the other side is full of dungeon specials that, not being able to claim, I can't USE. There is a magic door which is apparently invulnerable. There is a room full of knights I simply can't fight, because there's no way to attack them. (Not only does the barracks barely work at the best of times, again, it is across lava and there are no creatures available with ranged attacks that aren't just rebounded off, nor is there even a viable firing position anyway. Unless the intention is to micro-manage dozens of creatures one by one to be killed by the knights (or a level of expectation of FPS skill entirely unreasonable in a dungeon management game), there appears to be no way to fight them.

Elsewhere in the dungeon is blocked by more heroes, or walls; Destroy Walls, which from reading the comments on KeeperKlan (which did not help in the slightest and suggested that the needed bridge is neither past the knights nore the invulnerable door) is not located on your side of the lava, so without the ability to make a bridge, it woudn't be helpful even IF I found it.

Literally the only thing left to try is train a load of imps all the way up to get teleport which was vaguley hinted at on Keeperklan. Though how that would change anything I don't know unless it is some sort of coded event - since I just ran imps across with Haste and it didn't help. But in which case... We're back to that Sierra adventure-game logic. Why would I have started training imps on a map with no gems? With no time pressure and no gems putting emphasis on the aforementioned workshop economy, I mined everything out before getting any creatures, so there was zero point in training them and making myself more busy-work selling magic doors. And training imps now would be just more time sitting having to babysit that workshop economy than I'm prepared to waste.

I've been at this level for about, ooh, four hours. I think that is more than fair, and far more time than it deserves, despite the clearly complicated coding put into place. I am giving up. It's not worth the time of the frustration at this point (ESPECIALLY when I'm already extremely stressed out at KeeperFX is supposed to be the thing that helps me to relax and unwind.) It is, at the end of the day, just not fun.

(If, at some point, an actual solution is posted (here or somewhere and linked here where spoilers can be utilised which I assume they can't here), I might, if it does not require the investiture of too many more hours, come back to finish it, if I still have the saves.)

I am not going to give this a low star rating without at least giving the designer a chance to respond (as I say, bare minimum it is possible there's a bug introduced at some point with the complex coding via KeeperFX updates), especially as I don't know if I would be able to change it later. But at this current time, I definitely can't recommend it.