That's a pretty good first map, NicodemusRa :)

Only a couple of things I would recommend trying - to see if you think it improves the look or not.

Try adjusting the white glow around the map labels to make it smaller and more opaque, but keep the fade. It's not quite working to cut out the map detail right up against the letters. It's close, but it's not quite there.

Try reducing the size of the title just enough that the tops of the letters don't overlap the edge of the map.

A ditch at city scale is relatively easy because you only need to worry about the general appearance of the thing rather than adding too much detail.

Would something like this do?

That's a fractal polygon of the darkest grass shade drawn on top of the field with those three effects employed to give the illusion that it is sunken into the field. That's a dark glow set to outer, a pale shadow of the same extent, and an edge fade inner that is twice the width of the other two. You could use the same effect in any city map. I will find a way to leave it in the example map for you.

I've made it drain into the marsh and put mud in it instead of grass, so it makes sense in the map now

Remy wrote an article on how to show and hide secrets here:

You can edit the sheet effects to taste. Open the Sheets and Effects dialog again, pick the sheet with the effect you want to modify, and then pick the effect itself in the right hand pane. Then click the Edit button and a dialog will appear with all the settings you can adjust. Click Apply when you want to see how your changes affect the map.

Playing with sheet effects can take a while, but time spent playing will pay off in the end when you will be able to tell exactly how you want something to look and know how to make it look that way in the future.

You can switch on the sheet effects by opening the Sheets and Effects dialog |CC2SHEETS| and checking the Activate Sheet Effects box on the top right of that dialog. Click Apply, or Ok and you should see quite a difference in your map.

It sounds like you may have a line width set that is other than zero. Set the line width to zero and try again.

Don't worry - I forget sometimes and still end up drawing a line around an area instead of a filled polygon.

It looks pretty good for a first map. If you switch on the sheet effects those walls should cast shadows and feel a lot more like walls.