Fordham University has an impressive collection of history-related primary sources online. They are catagorized by time-frame - most importantly to you, at this point, is the Ancient History Sourcebook.
The Encyclopedia Brittanica, accessed through the Alabama Virtual Library, will provide classic encyclopedic information on your subject. A search "split roman ruins" returned a dozen articles from "Brittan 1450 to the present" to (simply) "Split (Croatia)."
Livius.org is a site created and (largely) written by Jona Lendering, a Dutch historian and educator. The work is comprehensive and scholarly.
History Study Center is one of the subscription sites the Upper School library pays for. Login and password are randolph / raiders. While you will find historical documents, study units, and journal articles, the best help will come in the form of maps and photographs.
UNESCO, the United Nations Educational, Scientific, and Cultural Organization, includes many gof the ruin sites on its list of World Heritage properties,"having outstanding universal value." This site has a wealth of information - cultural and historic - as well as photographs, an interactive map, and links to events, articles, and activities.