There are several differences between the editor in Stronghold 2 and in Stronghold Legends. For a complete overview, please see this article.
Firstly, one of the main differences is the fact that the estates do not need to be marked with a castle or village flag. This is much easier when making maps, as you do not end up with instant defeat. Instead, simply place a keep or village keep in the estate and that is the equivalent of the Stronghold 2 marker system. It is even easier than this too - the keeps now have a section of their own in a menu entitled "extra features":
On the left menu, from left to right are the following sections:
Another example of how the editor has changed is in the top left corner. From there, you can place buildings for each of the three game factions instead of simply one or another - this also changes the background colour of the section instead of leaving it as a single colour, as shown below:
Additionally, you can place units for each of the factions. The special units can be placed by going into the Units -> Arthur Shield/Ice Shield/Evil Shield section of the editor menu once in the editing mode of the map. Above the Arthur shield you can place standard units and below the Evil shield you can place siege equipment.
One of the other changes in the editor is that you can now place actual ruins in the editor. To do this, go to Extra Features -> Ruins. There are a selection of ruins to choose from, which can be placed on your map. In contrast with the Stronghold 2 editor, the Stronghold Legends editor is much more simplifiied and easy to use.