A standard and well-known explanation is that Takeda's cavalry, known as the most powerful, was defeated by the Oda-Tokugawa allied forces which employed as many as 3,000 teppo in the "sandan-uchi" tactics in which the gunners were arranged in three lines to compensate for the teppo's slow reloading.
織田・徳川方の3000丁という量の鉄砲、さらに鉄砲隊を3つに分け、鉄砲の弾込めによるタイムロスをなくす三段撃ち戦法で、最強と呼ばれた武田の騎馬隊を破ったというのが有名な通説である。