Should I make sure I can't attend now, so the stream will work this time 😊?

Have you tried drawing either a simple, quite thin, line, or segments (like logs), to make up a line? Use either black or a suitable log-brown colour. Booms would be typically made up of low-lying, floating obstacles, like logs, rafts, barrels, even ships, linked by lengths of chain under the water, and probably fastened at the shore ends by a boom tower, which would allow the chains to be raised or lowered to prevent or allow access, so you could add some of those floating objects along the line too. You could also have a couple of free-floating patrol boats alongside it for waterborne protection.

30 minutes! (With apologies to Sue!)

Ordinarily, if there were a lot of symbols there that your computer can't find, you'd end up with a load of shapes filled with red X's. As there aren't, that doesn't seem to be the issue here.

Have you tried doing a List command using "Select all"? That should tell you exactly what is actually in your drawings.

There are various possibilities for why so much seems to be missing from your drawings, and it may help if you're able to post copies of the FCW files here for others more expert than me on the Forum to examine, to try to identify what's going on here.

The problem my colleague was having after importing the JSON file into QGIS (free software) was that just the roads showed up. I was vaguely assuming that at least some of the available GIS software would have the topographical geoid data pre-loaded, but maybe that's not the case? I'm approaching this from the perspective of having no idea about GIS software in general though! Is there, for instance, a set of said topographical data available that could be imported into a program such as QGIS, over which the roads could then be overlaid?

In answer to Marja's point, judging by the discussion from 2023 on this Forum my first post here linked to, GIS data can't be directly imported into CC3+ or FT3, so could only be imported as an image that would then have to be traced.

Incidentally, a further colleague commented on that ancient history forum that it seems for Britannia, the Itiner-e map has used only the roads known and suspected from the original 1955 version of Ivan Margary's monumental work "The Roman Roads of Britain". This is odd, because there was an updated version published that expanded and corrected the earlier edition in 1973. I'd already expressed concern on that other forum, because I'd found at least two roads known to exist in Scotland that aren't on the Itiner-e map. In addition, it seems the Itiner-e map has excluded at least some British Roman Roads that don't connect into the rest of the network, despite the fact they're archaeologically attested. This could mean there are similar problems elsewhere, of course, for all it remains a fascinating resource map.

Plus of course on the snowy roofs, you'd really want to have the snow melted/cleared a bit round the chimney pots on the roofs sometimes too, and that'd be much trickier to manage!

You're not alone in battering away at unwanted nodes to get rid of them! I usually end up hiding or freezing any overlapping entities (like the LAND sheet here), and then just clicking to remove a node or two nearby more or less at random till something useful happens (and using "Undo" if it makes things worse!). Of course, the trick is then to unhide the LAND sheet and lose the same nodes in that, otherwise, as Sue said, the outline may no longer match the landmass...

Just time to generate a quick sample random city map before the stream (it may not take anything like that long, depending how long you take adjusting the parameters first)!