The mountains along at the western border of Kanagawa Prefecture draw the limits of the Kantō plain, and long represented an imposing barrier between Japan's old Kansai heartland to the west, and its later core – Edo/Tokyo – to the east. In the bottom corner sits the Hakone caldera, and north of there it is mountains, mountains and more mountains until the Sagami River and the pass through to Yamanashi.
Outside Hakone's northern rim runs 759m-high Ashigara Pass (足柄峠): a deep and narrow valley, and in old times the most accessible gap in this barrier. But with its susceptibility to eruptions of nearby Mount Fuji, and the rise in significance of Hakone and Odawara, this route was gradually eclipsed in favour of the Tōkaidō highway through Hakone itself.
You can read about that Hakone passage here, with its nasty Edo-period checkpoint. The mountains of Hakone's northern rim, Mt. Kintoki and Myojingadake and the ridge of bamboo between them, as well as Hakone's central lava dome of Kamiyama, also feature in that article.
Those are all on Ashigara Pass's south side. A different mountain stands on its north: Yaguradake (矢倉岳), a peak with a funny domed head.
Yagura (矢倉) means “watchtower” or “turret”. I do not know if such a structure once stood here, but you would struggle to find a better place to put one. Yaguradake's summit commands an open panorama, arcing from the Tanzawa Mountains to the east, across Odawara and Sagami Bay, then down Ashigara Pass and across to Hakone's mountains opposite; and finally round to the west, where it affords a remarkable view of Mt. Fuji and its surrounding lowlands.
|Facing southeast: the city of Odawara, and Sagami Bay in the background.|
|Facing south, towards Hakone. On the left is Myojingadake, and on the right Mt. Kintoki, joined by Hakone's northern ridge. Ashigara Pass is down below. Lurking in the middle background is Kamiyama, Hakone's central lava dome and highest point.|
|Facing west, towards Gotemba (Shizuoka Prefecture) and Mt. Fuji.|
To control this vantage point is to control Ashigara Pass. From here you can spot anything which tries to get through the ravine below, be they armies marching under rival warlords or disgruntled daimyō (feudal-era lords) rolling in from the west to invade the capital; or equally, any response therefrom dispatched by the shogun. From here your signals would reach far in both directions, across the uninterrupted flats, or even to ships out at sea. And there are only few ways up the mountain, each of them steep, dense with foliage and very defensible. And on top of all that, when it's someone else's shift, this site gives you the privilege of a sunlit nap on the grass or a classic picnic lunch with Mt. Fuji in the background.
This is a moderate hike. The way up Yaguradake is steep, but the ground is gentle, and it only takes an hour and half to climb at gentle pace. To get there, take the Odakyu Line to Shin-Matusda station (新松田駅), then go to the bus stops outside the North Exit and take the Hakone Tōzan Bus bound for Jizōdō (地蔵堂). (Timetable here: the correct bus is the one with 地 next to its times.) Get off at Yagurazawa (矢倉沢), where the route starts. The bus ride takes about 27 minutes and costs 660 yen.
After climbing the mountain, the fuller, most fulfilling route would then be along the forested ridge to the north and east, arriving in Yamakita village near the famous and beautiful Shasui Falls (洒水ノ滝); but a certain problem makes this route impassable at present, as we will see, so I cannot currently recommend that east trail. Instead you should go down the way you came, or take the other path down into Ashigara Pass itself, where there is probably a bus stop.