Any time you select a missile unit there will be a mêlée button that appears on the border of the circle that envelopes the picture of the unit commander in the lower left hand corner of the screen. It appears as crossed swords, and you have to highlight it for it to take effect. I could not find a toggle key command like those used for archer units in Med 2 and prior TW games.
I don't know how effective the Ashigaru bowmen would be although they do carry a sword. The Samurai bowmen would probably be better for this.
Cheers
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