If you want efficiency in making levels, you need a dedicated tool - you need to program a level editor.

No you don't.

If you want efficiency in making levels, you need to use an off-the-shelf editor, it will get you to a better level of functionality sooner. There are heaps of tilemap editors, and this game is mostly just a tilemap with various objects and metadata. It should be doable with minimum of custom code.

In order of preference:

* Use an existing editor

* Code a wrapper for existing code

* Code a plugin for existing editor

* Write your own editor

You'll have a lot more productivity if you use an existing editor, unless you have, say a team of at least 2 people fulltime generating content. And if that's the case, you won't be wasting time posting on gdnet.

If you want efficiency in making levels, you need to use an off-the-shelf editor

Hi. With that you will be efficient in making the level editor itself, not the actual content. Using something off-the-shelf is not a guarantee of efficiency. You will definitely save work from recycling the UI, navigation, I\O etc. but depending on the paradigm of the editor one chooses to extend, he would have so much work as to entitle making your plugin a standalone program in the first place - especially if it goes in a different direction than what the editor is intended for. The OP does not need refined functionality, but a convenient way to layout screen data.

Which leads to the following...

There are heaps of tilemap editors, and this game is mostly just a tilemap with various objects and metadata.

I don't think that is the style of game he wants to make. It's a 2D, first person dungeon crawler. Something like the following:

It's not a third person dungeon crawler.
If it were a tile-map game like that, he could indeed use one of the several tile-map editors available online (MapEditor for example).
Given that the style he's going for is very niche, I think a custom solution is, most likely, best.