Such an insurer shall not assume any risks on property situated within any one block of a closely built-up district or of territory within the limit of any incorporated city, unless on a single risk in such block it immediately reinsures all of the amount at risk which is in excess of the limit provided in this article or unless in the case of more than one risk in any such block it immediately reinsures all of the amount at risk which is in excess of one-quarter of 1 percent of its direct amount of insurance in force; provided, however, that any such insurer 90 percent of whose business in force is reinsurance assumed, and which has more than ten million dollars ($10,000,000) gross insurance in force, in the case of more than one risk in any such block, shall immediately reinsure all of the amount at risk which is in excess of one-quarter of 1 percent of its net amount of insurance in force.