That doesn't sound right.

Have you run Tools->Actions->Burn into surface?

It's a while since I wrote ODW, but I think this is necessary before you can do rivers well.

It may be that you have too much fractalisation going on, since those kinds of artefacts tend to happen when there are nodes too close together.

If you have thousands of nodes when you use Info List on the entity (just to make sure I'm right about that), try using the keyboard command SIMPLIFY to reduce that node count. Even if you use quite a small factor the command will at least remove the nodes that are causing the artefacts.

If SIMPLIFY is the answer, delete the wall or the floor that you didn't simplify, and use change like drawing tool to replace the missing part identical to the simplified shape.

To make something pop is to make it stand out, or up in the case of walls. Some people lengthen and deepen the shadows to make things look taller, though this can be a bit misleading because it makes the whole map look as if the building has no roof whatsoever. Some people play with the glows, especially a faint dark glow set to inner on the floors coming in from the edge, since it mimics the real world. Light never gets into the corners as intensely as it hits the floor in the middle of a room.

Oh yes - the furniture looks much more like its been battered around.

Arrow slits would be an interesting change from the norm, but either way.

Have you thought of making the walls pop a bit more by playing with the sheet effects on the WALLS and FLOOR sheets?

Create rivers in the normal way (Tools->Find Rivers) and tick the checkbox called 'Keep River Image Overlay' on the dialog that opens once the rivers have been calculated.

Once they are done, you can show the generated river overlay and edit it. This is not the same thing as the vector rivers already visible on the map.

And the only way to find the limits is by trying them ;)

I recommend running through the One Day Worldbuilder, which is included with FT3. That will allow you to take the FT3 world to a free app called Wilbur and bring it back to FT3 with decent river channels nicely eroded in the land. Otherwise you will probably end up with a mass of rivers so dense that the whole map is blue. This is what happened to me when I tried pushing everything to the absolute limits just now. I would say that is down to the lack of suitable channels.