Perhaps having a "draw Bezier curve" feature (similar to how you already have "draw line segment" and "draw freeform curve" features) would give the average user the roughly similar ability to make smooth-looking curves, without having to brush up on their math.

Done I still have some minor work to do on that feature, but the editor now includes a very basic "Create a Bezier Curve" possibility. With this feature, you'll be able to add perfect smooth curves to your Lemmings levels (as seen in the screenshots below).

Moreover, "Undo" & "Redo" functions are finished, too: so I guess I've finished the editor!
(... however grant me a little more time to clean the code before releasing the app ; it won't be long: sorry for the inconvenience, but I hope you'll be pleased with this level editor, and it will meet your expectations! Thank you!)

Thank you very much for your detailed answer!

Today, I didn't have too much time to spend on the editor, so I just took few minutes to add the "Shuffle" option: it randomizes the selected items' positions.
(the already-existing "Lazy" option randomly generates new items at random positions, whereas the "Shuffle" option doesn't create new items but allows you to choose/select level's items to be randomized: could be useful!)

New from today: the "Vertical Mirror" feature (= use of an horizontal axis for mirroring)
I coded the "Horizontal Mirror" feature early in the development, so I definitively should have done this one earlier

TODO: I have to work on the "Undo/Redo" functions, which need fixes.

(scene made in seconds using the lightweight GUI only + snapgrid + "Brush" mode + "Vertical Mirror" feature, all of them triggered by keyboard shortcuts)

I really wanted this editor to be usable by the maximum number of people, so I had to work on the GUI's size, and I've just finished working on the ability to resize the GUI (and that was quite a work, actually ).
Now the level editor could virtually work on any size of monitors: so whatever size is your computer's screen, you will be able to fully use the level editor The config file is initially set with relevant data to launch the editor using large width & height. But you can freely edit the config file to set a smaller (or bigger!) size for the GUI, depending on your needs.
You can also arrange GUI's layout as seen on the screenshots below. And actual level's size is also configurable (in order to make levels for other games than Lemmings).
And, finally, for those who mainly work with keyboard's shortcuts (which are now freely mappable, by the way  ) and want to work with a lightweight GUI, then, you'll also be able to mask the bottom part of the editor (of course you can easily make it re-appear by a simple hit on the same key that made it disappear): this is a nice addition.

- arranged layout, with edition tools next to the level:


- standard layout for small screens (yes, yes, this is the full view of the editor! Notice the appearance of an horizontal scrollbar at the very bottom of the GUI, to access all the available edition tools/features):


- standard layout for big screens (all the available tools/features of the GUI are visible on screen)


- standard layout for big screens, lightweight GUI (bottom part masked, so you'd better have to know your keyboard shortcuts ):


By the way:

Everything should be freely mappable to user-definable hotkeys.

done.

how compatible is your program right now? For example, If I were to make a level with it, save it as INI, could I then open the level up in Lemmix and it would work?

Yes, it works

Yeah, the formula editor is a bit of advanced feature but I'm glad it's here
It uses Javascript's functions/syntax, and formulas should result in a single number.
Moreover, I'm, too, interested in having Bezier curves in the level editor. Maybe one day...

For making things easier with the formula editor, I've already pre-entered some common functions (as seen in the screenshot below).

below: Making a random (but coherent) ground in seconds using the formula-editor (0 <= Math.random() < 1)


I have also made the formula editor more powerful than yesterday by enabling the use of items' attributes in formulas: for example, now, you can apply changes every n items, or on Terrains/Object/Steel only, or on items with specific width/height or position, etc... (ie: in the following screenshot, I skip 4 items before applying changes on item's position using an incremental variable on Y).

Cool things have been done on the level-editor recently

- one can load & save Lemmings' .lvl files (as well as Lemmini's .ini files and editor's native .xml files).
- steel/metal pieces are 100% handled now.
- the snapgrid displays units in pixels (on x and y).
- you can filter what kind of items are displayed on screen (ie: all items appear in the 1st screen, whereas the 2nd screen displays Terrains only: no more yellow arrows, see?)
- I've also added the "visibility mask", which turns gray everything which is out of ingame screen's 640px-width limit (very useful when setting your level's starting point, for example). This feature is demonstrated on the 2nd screen.
- etc ...



And you can also use another mask-related feature to make quick and easy object alignments against mask's borders ("Collision Mask" feature).
In the screenshot below, top-left: the initial scene, top-right: I defined my mask, and selected some items, bottom-left: perfect left-alignements, bottom-right: perfect right-alignments.

I've just added a new feature that will help to accurately design the levels you imagine: the "Path Mode" feature.

In this new mode,
- 1) you select a single entity or a Favorite made of several entities to be cloned/generated along a path (in the screenshot below, I selected a multi-entity favorite => it's the red-selected item in the item list, and on the level's left side, you can see this favorite is not a single object but it's actually made of some various items)

- 2) then, you continuously draw each segment of your path until you decide to stop your path. And for each single segment, you set in real time the number of clones you want along the current segment (using keyboard's arrows to adjust the number of clones).
During this process, you see in real time the location of each item/clone on the segment: so, as you stretch and move path's current segment, you have an instant visual feedback of how your items will be finally set in the level. This is very coooooool to accurately design level's areas!
And until you commit the path's current segment, the clones that are displayed on screen act only as visual references (therefore, you don't see them in the minimap, since I didn't commit the path yet). When you commit the segment, items are actually added to the level.

Paths can be snapped on the grid lines too

See this screenshot:
- 1 single segment,
- 3 clones wanted in the 1st screen,
- then, 14 clones wanted in the 2nd screen (very same segment)
- and, finally, 12 clones on a shorten straight horizontal line, in the 3rd screen
=> real-time visual feedback of what one is about to draw in the level



And the "Path" feature isn't limited to straight lines: you can also perform freehand drawings. Moreover, when you use curved paths, you can specify in real-time the spacing between each drawn item, and, as with straight-line paths seen above, you have a real-time visual feedback of what you're doing: veeeeeeeeery handy to design cool things!

So, with this new tool, generating landscapes is a very easy -and almost instant- job! It's really cool

Please, see the screenshot below:
- same path used on each screen (blue hand-drawn curve),
- selected entity used for the path is a Favorite made of 4 items
- spacings used are 1) 6-unit spacing (continuous landscape), 2) then 17-unit spacing, 3) then 45-unit spacing, 4) and finally I commited my path using a 99-unit spacing which results in a landscape made of individual islands.