-atum is a suffix used to create a noun from a verb. -acum is a suffix in latinized forms of certain early Celtic place names, the meaning should be a sort of locative or a noun created from another word adding a "place of" meaning.
I love this mod, and have for a long time considered making something similar myself. Creating place names aren't really that magical, but understanding how humans have created the names we use today through history I find really interesting.
This is not meant as a criticism but more a comment, the names created tends to be a bit long. Especially early place names are usually just simple descriptive words of the landscape and if there are lots of similar nearby you get some additions (ex. Valley would be to generic unless it is something unusually valleyish which deserved it to be "The Valley", hence you could distinguish two valleys by names like "Redvalley"/"Greenvalley", "Sunvalley"/"Coldvalley"). That is the reason you have lots of same short place names. And if there are more than 3-4 syllables they will be shortened in some way. How this shortening affects actual place names depends also on whether you are in a society of primarily oral traditions, as writing the number of syllable is less of a concern. However when society use writing, there may still be a longer "formal"/"upper-class"/"official" long name, which will have an "informal" version in speech or an abbreviated version for writing.
When a place naturally becomes a city or town (also here definitions vary wildly about what consists a city and also opposed to a new settlement founded by a leader of a group of people) they may retain the original place name, but during time some distinguishing features about the name would be added which the inhabitants identifies with and distinguishes from nearby places.
The early "cities" were group of people who lived closer together, and formed some structural society when getting larger. When they formed defensive structures which define an area you get usually the names with endings like -bourg, -polis, -puram etc. The new names may not retain anything about a former place name or geographic description however. Depending on the circumstances there will be a new name which could be derived from a name of the current leader, former hero, god/godess, or some other unique feature. The given name by the locals may be different to what the neighbor calls it, and thus if their neighbor conquers the city could add some negative connotation through the name, and as a new leader the de facto name of the city becomes what the conquerors referred the city as. External effects may also name the city from certain produce or profession the inhabitants holds. Different languages and phonemes will change this names as well.
About the word lists, it is important to recognize that many place names in Northern Europe are in the early days based from Celtic names, Germanic, and then later either latinized or received new Latin names, before the more modern current local languages. In Southern Europe you had Etruscan and Greek names before latin. In the Levantic Middle East you find names from Sumer, Aramaic, Greek, Egyptic/Coptic. etc. and in the east early Persian which is similar to early sanskrit. And for the southern asian area, and south-east Asia Sanskrit have been the main source of many many names, combined with their earlier indigenous languages. The same is valid everywhere. Thus a wordlist should either hold a historical context, or only reflect the contemporary language.
