As an aside: I know of at least one level that made deliberate use of multiple talismans being concealed, by having a bronze talisman requiring saving X or using at most Y builders, then a silver or gold talisman for saving even more or using even fewer. The intention was that the player would challenge themselves to solve the bronze talisman first, then have a "holy ****!" moment when the game informed them that the more stringent requirement was achievable.

This shouldn't be dismissed, but there are two big problems with leaving things as they are. Firstly, this intended effect falls completely flat when the game doesn't inform the player about the second/third talisman. Secondly, a level may have multiple talismans that are not hard-and-harder versions of the same challenge, but orthogonal (e.g. one for saving X, another for at most Y builders). In the case the player would benefit from learning about the talismans at the same time so they could choose which they prefer to work on first.

Can someone remind me some details of the current talismans system?  It sounds like you can create as many as you like, and classify each independently as gold/silver/bronze however you want?  Or is it more like a one gold, one silver, one bronze system (probably not, since I'm not getting any sense that there are always 3 talismans)?

Talismans are basically NeoLemmix's version of achievements, and across many different games I've seen all kinds of variations on how achievements are shown or not shown.  I do think the safe default is to be able to just show them all on the outset (ie. user can at any time find out exactly what they are in terms of requirements).  A progressive-unlock system like Proxima mentioned in the aside is interesting and not uncommon amongst other games, but I still feel like it's more common that no achievements are hidden on the outset.  Games using progressive-unlock achievements are also generally very systematic about it--usually each different kind of achievement (eg. "solve X puzzles without requesting hints", "solve a puzzle under N minutes", "find Y of whatever scattered throughout the game", etc.) each has M levels (eg. bronze, silver, gold) of achievements.  Some games also have a few to many completely hidden achievements where the player is never informed they exist nor what the requirements are beforehand, and only find out about one upon achieving it.  I'm not generally a fan of that and so I guess it's good thing that you can't really do that currently in NeoLemmix.

Most games have a relatively easy way to review achievements while in-game (and by that I really mean "in-level" or similar).  Either there is direct access to it (ie. a hotkey and/or some UI element you can always find and click on to show the list), or you go through the main in-game menu to get to it.  Most games also notify the player immediately in-game (typically via notification-style flyouts that are non-modal and disappears on its own after a few seconds) whenever an achievement is achieved, though I think it's fine for NeoLemmix to not be that immediate and just tell the player about it on the postview screen or similar.