Spies
For tactical intelligence formal spies were employed. Once war had been decided on but before mobilization, spies (quimichtin, literally, mice; sing, quimichin) were sent into enemy territory dressed like the foes and speaking their language, but they traveled at night and tried to remain hidden. Their job was to observe the enemy's fortifications, army, preparations, and so forth. They also sought out the dissidents present in virtually every land and paid them for information. Maps were then drawn of the territory to be crossed, and obstacles such as rivers were marked.
The use of spies was a two-edged sword, however, and Tenochtitlan was also penetrated by disguised enemies, foreign merchants, disgruntled "allies," and domestic traitors. If enemy spies or local informers were discovered, they were tom apart, and their families were enslaved. The same fate awaited Aztec spies who fell into enemy hands, so spies were well compensated (usually with land) for the danger of their work. But plans could not be kept secret forever, particularly once mobilization had begun. (Ross Hassig, Aztec Warfare: Imperial Expansion and Political Control [Norman, Oklahoma: University of Oklahoma Press, 1988], 51-52)