From their lofty summits overlooking Boston Harbor and the city itself, the colonists can fire cannonballs--which is why the British have spent the last three weeks secretly plotting an invasion of the Charlestown Peninsula, to capture those heights. But just two days ago, on June 13, the rebels got word of the British plans. Working day and night, they have been preparing redoubts from which to fend off the redcoats. Six feet high, made of earth, with wooden platforms on the interior from which men can stand to fire their muskets, the square-shaped