(trying the navigator version)

Semi-on-the-fly remarks:

1. Your justification for the constraint is intellectually right, but at the end of the day… the concept of the game is basically tower defence — unless I missed something. ;)
   (I also notice the game is tagged as ‘Point & Click’, but this is meant as the adventure subgenre rather than the mere mouse interface style.)
2. I like the characters, especially the knight; and the zombie looks like he comes straight out of Minecraft. X)
   Where is the axolordl, by the way? Maybe is it his laugh we hear. :)
3. Showing prices along the buttons/interface would help enormously. More generally, I think putting indications would help, such as the name of the rooms even when not clicking. At first, I had a hard time getting what was what, how much it cost, what I could do, and so on.
4. The Settings button does not work?
5. Trying to apply a second speed upgrade to the gold mine resulted in a crash with a very lenghty error.
6. My understanding is that gold is pretty much physical power (creatures), while gems is magic. (By the way, gold+gems remind me of Dungeon Keeper — which is only storywise connected to the game. X) The heroes too: knight, wizard and possibly(?) thief.)
7. I tried Dark Aura and did not see the effect; does it decrease all heroes’ health?
8. The upper-left room has no text when clicking. Clicking on its left button results in another crash.
9. It took me some time to understand how the sounds were linked to the action! Maybe some indications (whether visual — an arrow, particles… — or textual — ‘A goblin spawned!’ —) would help.
10. I realize that since the heroes are in limited number, they would have to gain some power to still pose a threat; I am not sure if they become more powerful… Is this the case? If so, an indication of their power would be great.
    At the moment, it looks like the heroes cannot win even if the player does nothing. (Which leads me to a question: what happens when you lose?)
11. Upgrading the skeleton generation speed causes another crash.

Conclusion: the main concept is classic (although it mixes long-term investment and short-term powers/abilities), and the constraint here is a small variation; the characters and effects (magic, sounds…) are cute. I think expanding the interface (indications) would help the game get more playable, as I found it hard to discover what is going on! On the gameplay side, you could make the most of the loop constraint by strengthening the persistent heroes (the game I tried just before yours, Final boss vs hero, actually uses this idea). (As for the bugs, they may be caused by the navigator version? I do not know.)

Kudos for the visual setting, including those musket-wielding wizards! XD Sounds like a magic-mechanic hybrid class from Arcanum: Of Steamworks and Magick Obscura gone wrong. ;)

(Game not tested yet.) Just to let you know, the game page tells me on the top that no file is available, which apparently means that you set up your project as having a navigator version, without a navigator file (or without a proper one?).

PS: seeing at the bottom that you did provide a ZIP navigator build, you might have missed the checkbox to enable the file for navigator launching.
PPS: on second thoughts, no: I see the ZIP is the desktop version, so the navigator one must be the other, titled ‘https://simmer.io/@tanruss1/tower-of-darkness’; the URL does work, by the way, but if you want the itch.io player to work, then a ZIP or HTML file should be provided. Nothing serious as your game works!!