Fort Koyasan is a Toyotama fortification where Ryuzo's men gather to flank Lord Shimura's army. You attack it in A Reckoning in Blood before the siege of Castle Shimura.
Lord Shimura learns that Mongol forces under Ryuzo are stationed here and could hit his army from behind. Taking the fort gives him the edge he needs for the castle assault, and gives Jin a chance to face Ryuzo.
Yuna refuses to join, wanting to leave for the mainland. Taka turns up anyway and offers to distract the guards at the entrance.
Lord Shimura's Camp
Lord Shimura's Camp is the samurai war camp in Kubara where your uncle gathers his army to retake Castle Shimura. You meet your allies here before A Reckoning in Blood and The Fate of Tsushima.
Akashima Village
Akashima Village, in the prefecture of the same name, is the starting point of Norio's storyline. It has a full set of vendors, so it is a good resupply stop in western Toyotama.
Bamboo Forest Hot Spring
Bamboo Forest Hot Spring sits in Kushi, part of Toyotama, and is one of the game's 20 Hot Springs. Soak there once and Jin's maximum health goes up permanently.
Castle Shimura
Castle Shimura is the seat of Clan Shimura in Toyotama and the biggest battle of Act 2. Khotun Khan holds it after the invasion, and you retake it in The Fate of Tsushima alongside every ally you have gathered.