Is doing the Orc Village necessary for the game to progress?
I'm right after you finish the boss of the forest s of 4th town. Right now, the bridge to the Orc forest opened up. However, if I enter the 4th town to buy the runes from the shop that just now opened, the bridge breaks the moment I leave town. Did I do things in the wrong order? I can't survive the slimes guarding the water cave if I don't upgrade my runes, but I'm concerned about locking myself out of the Orc camp.
Technically the Orc Forest isn't necessary for game progression, but you do need to get yourself involved to recruit Mochiko who, like every single other possible party member, is pretty much indispensable if you want even a sliver of a chance to realistically defeat the final boss NG0, and the book for Steel smelting is in the very end.
On the other hand, skimming the enemy dictionary says that you're not really supposed to be spelunking in the Orc Forest until after the Snow Cave (full Iron/Black Iron/Cast Iron and Lv3 Crests) at the very least, so maybe you could keep a separate save and just progress into the Water Cave -> Snow Town/Snow Cave and see if the bridge comes back afterward? I dunno. Prolly a bug.
For the record, a very abridged barebones game progression checklist IMO would be:
-1st village & surrounding area
-west forest
-2nd town area
-lord mansion visit 1 (to unlock Salt for sale)
-3rd village & surrounding area (for boat)
-4th city
-King Castle/Tower of Trials (for ship)
-Bronze Mine (Cupronickel smelting)
-Skill Temple (can be done a bit sooner or later though)
-Tentacle Jungle (Lv3 Crests)
-Underwater Cave (Iron smelting)
-Snow Town/Cave (Magical Ruby)
-Orc Forest (Steel smelting)
-Floating City (Wind stone for airship)
-Destroyed Town (Silver smelting)
-Forest of Eternal Night (Secret Key for Lv4 Crests)
-Maou's Castle
-Abandoned? Village (for pickaxe)
-Volcano (for Dragon Dive)
-Tentacle Jungle Depths (Darksteel/Magnetite/Cobalt smelting)
-Forest of Fairies (Hero's Inheritance, Sword Saint's Crest)
-Tower of Trials Basement (vs Titan for Lv5 Crests)
-SE island of worldmap (vs Old Beast)
-SW cave of worldmap (vs Evil Source)
-Lead Mine Depths (vs Old One)
-Floating Castle (vs Omega)
-Final Boss Fight