Getting there

There are a few ways you can get to this hike. First of which is taking the Akechi Tetsudo line from Ena station to Iwamura station and then walking to the castle. The other is by driving to the 第一駐車場 and walking to the castle ruins. You alternatively can just drive to the base of the castle and walk from there but that is no fun. I recommend walking from either the first carpark or the train station as to get to the castle you have to walk through the old historic town centre which is ladden with buildings from the Edo period and prior. It has a really cool vibe which is arguably better than the castle itself. 

Maps

Google Maps

Route/Trail notes

From where ever you begin the hike from, I recommend first plugging into Google Maps, 岩村城下町 and having a waltz up and down the mainstreet. After walking from the bottom of the hill to the top, you should pass by a sign that says in both English and Japanese, Iwamura Castle Ruins. Follow this small sign through a backstreet. The path follows the road and will take you over a small bridge and past Iwamura Primary School. Continue along this road up a hill where you will see the base of the castle and some castle ruins. Do not enter here as this is just a carpark. Continue going up the hill where you will some stone stairs in a forested area. Walk up these stairs for about 30 minutes and you will reach the Honmaru (castle keep) at the top of the mountain. From the castle there are limited views of the surrounding mountains. On a clear day you can see Mt. Ena. The only thing remaining of the castle are parts of the wall which are lathered in green moss. It looks pretty cool.

There are also some museums knocking about the area if it tickles your fancy, just don't rely on them to have English signage. There is also an Italian resturant (Green Herbs) in the old historic centre that is run by an Australian guy. You can drop in and say g'day. I'm sure there aren't many Aussies knocking about rural Gifu. 

Iwamura is also home to a famous sake brewery called Onna Jyoushu 女城主. You can find the brewery in the historic centre. Across from the sake brewery is a castella cake shop which is also famous in the area. If you want to be a bit more daring, you can try some of the local Ena and Gifu specialities including Gohei Mochi which is a delicious peanut based savoury rice dish on a stick. There is also hebo which are wasps. These taste better than they sound. 

From the peak of the mountain, head back down the same way. 

Permits/Costs

Just train fare

Other References

Good luck finding anything else mate