No, you don't need to download the public mod, although it is one solution. The alternative is to reset the svn copy to the state of the original repository (thus, the one WFG hosts). In both cases you'll loose your own changes, but not any newly added files. Because of that it is handier to work in a separate mod folder. Any file found in a mod overwrites the data from a file in an earlier included mod. So let's say you want to mod simulation/templates/structures/egypt_mill.xml, it is best to duplicate that file in a mod separate from the public mod (of course, file structure needs to be similar, so the duplicate would go in yourmod/simulation/templates/structures/egypt_mill.xml). In this case though, all you might have to do to get rid of the error is to open the egypt_mill.xml and look for the parent XML file it points to. Without it its data is incomplete as it inherits data from that parent xml file, hence the errors. Simply put, the game engine takes the file, looks for any parent, takes that data if there is a parent xml file and overwrites any values from the parent with those in the current xml file. In this case the parent info should say: <Entity parent="template_structure_economic_storehouse">The name of this xml file doesn't matter much for the current error. Of course, any unit that should be able to build this structure needs the correct name