M.L. West (1937-2015), The East Face of Helicon: West Asiatic Elements in Greek Poetry and Myth
(1997; rpt. Oxford: Clarendon Press, 2003), pp. 455-456:
Seven champions simultaneously assault the seven gates
of the city, where each is faced by a matching champion and laid low.
This is obviously an artificial scheme. It is unlikely that any Mycenaean
citadel ever had seven gates. The principles of fortification were based
on the restriction of access points to the minimum. The hill on which
Thebes lies has only three natural approaches. The late Bronze Age city,
in the view of archaeological experts, can have had only three or four
gates. Even if there had been seven, what general, supposing he
happened to have precisely seven heroes at his disposal, would divide his
forces equally between the gates instead of concentrating them at the
weakest point of the defences?