Population, Pre-History, and a Lost Continent

Just playing with a few ideas:

• Total population in 1750 is far less than Europe's - say about 40 million as opposed to about 120 million for Europe.
• Our seed European population in the 1170s is a couple of thousand, of which only a couple of hundred are nobility..
• but including a wide range of trades and skills.  Clearly, their original plan included some aspect of colonization.
•  and aware of iron production and three-crop rotation.
•  There is a base population on the island; exactly the size I will have to work out
• Bronze age technology
• but superlative sanitation in their near-abandoned ancient cities.
• and major domestic animals along with the rest of the  fertile crescent crop package, especially wheat.
• The local nobility is rapidly displaced (and merged) with the European
• agricultural yields go up with improved crop rotation and iron-shod plows.
• the excellent sanitation is adopted with enthusiasm by the Europeans
• leading to a burst of population growth, although probably still with a preindustrial population curve. 

Geographically, I am thinking of a substantial landmass of varied geography  surrounded by something of an archipelago.  Many would be obvious former volcanic calderas, such as scaled up versions of this small Hawaiian island with fertile areas generated by extended erosion. This lets us have significant Naval operations; but still the area we have is large enough that some at least would be isolated from maritime interference.