Apart from snapping to 1st and 2nd helper; you can also snap to any corner of any object. You can also snap to an intersection; provided that the intersection point belongs to the current edge (i.e. the line segment where the 1st and 2nd helper is currently placed)

This snap is done within the tolerence determined by the size of the architect's square shape. Sometimes there may be more than one corner/point that it can potentially snap to. Once the architects gets snapped to one of those candidate points, you can make it snap to other candidate points by pressing the hot key: Ctrl+dot (i.e. press the Ctrl and the Dot key together)