[jester]

Build a comcrete pier and breakwater so you have your own little protected harbor, add build an artificial reef too, so that only you know the route, or if its know so that anyone entering needs to go slow thus allowing you control and initiative.

One storey structures! Why one storey? You could probably make up to 3 and 4 storey structures. Remember alot of the temples and other great buildings of antiquity were damaged and looted for their stone as building material for other structures.

A protected coral, figure enclosures 12 feet tall to hold sturdy animals like bison or elephants, imagine the steaks you'd get from those critters?

A series of trebuches to use the ruble as a long range defensive weapon. Remember, siege in the Lord of the Rings "Return of the King? Big ol stone throwers like those/

Make some raiser platforms like zigarots just for the hell of it and let the archeologists figure out their meaning 1000 yeaers down trhe line.

Use them as a matrix making hollow squares out into the river and then using them to collect the silt, this growing the island, and of course filling in the hollow squares with waste from your community and industry thus having a square garden plot of river silt which makes good farm land.