Hi! Honest answer: there's no visual editor, but most of a full game here is data, not code. Heroes, monsters, waves, survival maps and story chapters are entries in src/data.gd where you change names and numbers, the dialogue and quests are plain text files in story/, and you swap art by dropping PNGs into art/extra/. The docs folder has a page for each of these with examples.
Where you'd write a little GDScript is a brand-new weapon behaviour (5 to 15 lines, copying one of the 14 that are there) or a new kind of map layout. So you can build your own game with your own heroes, enemies, maps and story mostly by editing data, and only touch code if you want new mechanics.
The free sample on this page is the full game exported from this same project, so you can play exactly what you'd be customizing before you buy. And if you get stuck on something after, ask here or on our Discord and we'll help.