Dungeon Mastery: Ruined Castles in Anime

Castle of the Slime Wizard

Two different castle ruins to be explored by the heroes in isekai anime appear in the Anime Isekai wa Smartphone and Konosuba.
In Isekai wa Smartphone a castle sits on a lake at the end of a well maintained road. Here is the ruined castle of a dead Wizard who specialized in evolving slimes and jellies into Mimics- prefering to have them assume the shape of pretty girls. Of course the OP and support harem enter the castle and are menaced and generally endangered by slimes and Mimics.

Fortress of the Devil-king's General

In Konosuba the Protagonist hikes way out into the middle of nowhere with an Arch-Wizard armed with Explosion magic so she can get some practice. They find a castle and Arch-Wizard Megumin nukes it with an Explosion spell from great distance on a daily basis.
The Headless Horseman who to the heroes unknowingly resides there is a General of the Devil-king's army and is backed up by an army of Undead Knights.

The difference in series is that these ruins vary in accessibility. One is at the end of the well maintained road that leads to it, the other overlooking a rickety track that might have once serviced the citadel.