Thank you for playing my game and especially for the feedback. First of all: I should have added a pause screen to the recipes. I was clearly biased as a Dev, knowing the recipes already, towards thinking that not pausing the game is fine. For the invincible enemies: yes that is a bug. There is a script attached to them which is supposed to enable some scripts while they are visible and disable the same scripts when being not-visible. For whatever reason this script can break. The fix was to disable the "disabling-mechanic" so that once you see an enemy they will just follow you. However: I sadly cannot patch the game right now.
Finally: clever observation. The difficulty is bound to opening locks. I did not want to make this public inside of the game, so that the player has the opportunity of discovering it. But it seems like it is more frustrating than interesting!? Maybe a warning would have been useful like: "Do you want to increase the difficulty?". Under the hood there is a "world-tier"-system which controls which events are spawned from the clouds populating the game world. Depending on the current world-tier the chance that more challenging enemies occur gets increased. More or less you had the optimal experience of figuring out that you can start by building some towers, to get a "stable" defense and then discover the world to get enough buffs and weapons to defeat the boss.
Once again: I appreciate the feedback. If there is anything confusing, please let me know.