It's not clear from the op or the post with the terrain instructions if the limitations in 0.73 are now corrected.
- Can rivers be placed?
- Can forests/jungles be placed?
- Marshes?
- Are the other standard Firaxis terrains stable?
I'm ready to start detailed work on the map for the Asoka scenario. Basics were done with bmp2bic. These are the main things I'll be doing. If the answers are positive I'd like to give the map editing features of this program a workout.
You're right, the first post is slightly out-of-date in that area. At this point, the map editor is fairly stable as long as you stick to the standard Firaxis limits (i.e., don't try to put jungle in deserts). Specifically:
They can, although the method is somewhat different than in Firaxis's editor. In my editor, you select a tile, and then use NW/NE/SW/SE checkboxes to indicate where the river should go along the tile's borders. This will be slower than Firaxis's editor in some cases, but allows more precision control.
- Can forests/jungles be placed?
They can. Currently the editor does not limit you to only putting them on sensible tiles. But as long as you only put them on tiles that the Firaxis editor allows, you should be fine. I can probably add a limitation to "sensible" forest/jungle placements to 0.79 or 0.80 (not sure if I'll have a 0.79 before 0.80 or not).
Pine forests are not currently available, although any existing ones will display properly. They likely will be added in the future (along with snow-capped mountains). Also note that mountains in the jungles won't display with jungle on their sides in the editor, but they will in-game.
Those also work, in the same manner as forests and jungles.
- Are the other standard Firaxis terrains stable?
As long as you stick to what Firaxis's editor allows in terms of not putting forests/jungles/marshes on terrains that they can't normally be on, the terrains are to the best of my knowledge stable. The last bug that I remember that could render a plain map unusable was fixed in version 0.71, according to my changelog. Subsequent versions have fixed bugs not directly related to painting terrain.
The only bug that I can think of right now with regards to standard map editing is that if you go to add a city, but click Cancel when you have the prompt to give the city a name, the map editor crashes. This will be fixed in the next version. The current workaround is that if you accidentally add a city, go ahead and give it a name and then delete the city.
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There's some other graphical deficiencies, such as that river deltas don't show, that appear in the editor itself but not in-game. These are considered somewhat low priority since they don't affect the game and don't make it difficult to understand what's happening on the map.
At this point I am considering making the next version 0.80, with the map editor enabled by default to reflect its considerably more mature state than in the early 0.70's, and thus signifying that it's ready for standard use like the other tabs. I'd also like 0.80 to have several new features, though, so 0.79 may come before 0.80.
Landmark terrains are a work in progress. Currently, you can't add them to the map. That will probably be fixed in 0.80, but they still need some work around the edges. Since they can't be added now, they aren't a danger.
Also, if you plan on adding lots of buildings to lots of cities, version 0.80 may be of interest. Not to spoil too much here, of course

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I am curious to hear how horizontal mousewheel scrolling works on Macs. In the worst case, if it causes erratic scrolling, it can be disabled again in Settings. So far no one has reported back either way on whether it works on Macs. I'm also looking forward to hearing how the map editing goes in general. I've mostly been trying to get the editor to break, rather than editing realistic maps.
Despite it being fairly stable in regular editing, I still recommend testing in Civ3 fairly often to make sure the scenario is still working, and making backups. By default the program will auto-save a backup every 4 minutes, with up to 10 backup files, but both of those can be changed in Settings if you prefer a shorter interval or a longer trail of backups.