The mention of Echigoya's name by Tajima in the bamboo forest was proof that Tokubei had inherited the family business after his father was killed. You must infiltrate the Echigoya estate and punish the evil merchant.
Execute the Evil Merchant!
Objectives: Trust your ki and avoid being spotted. Find items to help you on your mission. Archers can see farther than other guards.
You begin on an exterior wall overlooking an inner courtyard. Drop down to the right and pick up a box containing a health power-up. There's no guard patrolling down below. It's fairly easy to remain on this roof and eliminate the guards in the area. Go left (note the geisha below, but don't kill her) and around to the right until the roof ends just before a stone ramp formation. Grapple up to the formation. Don't drop down, or a nearby guard dog may detect you.
Ascend the ramp, then drop down behind a guard below (he's at a locked gate you'll need to bypass later). Return to the rooftops and seek out the guards patrolling the inner area. You can avoid the locked door altogether by entering the building within the courtyard you started near. Enter this building and find a tunnel in the back room. Maneuver through the tunnel to bypass the gate into the next area.
If you decide to go the gate route, you'll need a key. Exploring reveals three jovial guards protecting the gate's key. Go to the opposite side of the courtyard to find their building. Sneak up on the nearest guard and perform a stealth kill. Flee, and when the other two end their pursuit, sneak up and stealth-kill them. Grab the key and go to the gate. Unlock the gate and proceed through.
Continue through this tunnel and stop at each corner to look for guards. You'll find a couple of guards in conversation. Hide around the corner, and when the nearest moves toward you, perform a stealth kill. Eliminate the other when he's walking away.