Expanding on and combining Omni's and Amorphous explanations, a Tell city is indeed the term used for cities where new constructions were built and rebuilt on top of the remains of the older cities, thus effectively forming a hill in a formerly flat landscape.
A famous example is the Mythological Troy which, if I recall correctly, was found to be on the 7th or 9th layer of out of a total of some 20-odd cities built on top of one another. Can't remeber whether it was 7th/9th from the top or the bottom though.
This is a very common occurence throughout ancient times in the Middle east, starting from the very first cities built (All right, from the very second ones, which were built on top of those very first ones.) untill... and this is where my historical knowledge fails me, but if I were to hazard a guess I'd say till roughly 300BC. (I can imagine the various Persian civilisations still doing it, but Alexander had a penchant of building brand new cities, possbily next to older ones, but not on top of them I believe.)
And this concludes today's lecture...
