How the French commander came to make this choice we shall discuss in its proper place. Suffice it to say that Wellington had not realized how bad was Masséna’s information, how worthless his maps, and--what is most surprising of all--how entirely destitute of local knowledge were the Portuguese traitors--Alorna, Pamplona, and the other renegade officers--whom the Emperor had sent as guides and advisers to the Marshal. And in truth, the unsuspected ignorance of Masséna and his advisers added an incalculable element of chance to the problem set before Wellington. He was obliged to make his plans on the hypothesis that the enemy would make the correct move: and not unfrequently the enemy, for reasons which the English general could not possibly foresee, made the wrong one. [159] There were some others thrown up on the extreme lower course of the Zezere, by Barca Nova and Punhete, to guard against a possible but unlikely use of the Castello Branco road by the enemy. [160] Wellington to Hill (_Disp._, vi. p. 441), Sept. 15. [161] Wellington to Chas. Stuart, Sept. 18. The French invasion was bound to commence with a preliminary clearance of the outlying fortresses still in the hands of the Spaniards. These to some extent protected the Portuguese frontier in 1810, though they had been built with the express purpose, not of protecting, but of threatening it, and had never before been attacked by an enemy coming from the east. Only three of these fortresses were of any importance--Astorga, Ciudad Rodrigo, and Badajoz. The other strongholds of Spanish Estremadura, Alcantara (which had stood a siege in the War of the Spanish Succession), Albuquerque, Olivenza, were either not in a state of defence at all, or were hopelessly antiquated, and little suited to face modern artillery and modern siegecraft.