I've updated the link to the mapeditor to load the new version 0.9 of the mapeditor today.
All maps from earlier versions will need to be loaded and saved in the new 0.9 format since most terrains has changed.
We haven't updated the link to list maps so you can't see the submitted 0.9 maps yet. It will change in a couple of days to list 0.9 maps instead of 0.8.
We will send a mail to all map makers in a day or so to remind them about updating their maps for the new version.
Maps that haven't been updated to 0.9 in a few weeks will be updated by us.
There are some pretty fun maps that haven't been updated to new versions of the game because the creator stopped playing Warbarons. We will try bring them to this version and open them for anyone to continue work on. For example Pillagers Westeros map would be fun to see updated.
Before you spend hours on you new masterpiece try to create a basic map and save / load it to make sure there are no undiscovered bugs that could stop you.
One more thing. The new river terrain needs to be painted on other terrain, if other terrain is painted after the borders will cover the river bank. You can always repaint the river again to make it look right.