A sequel of sorts to Sabre Wulf. But it's like the difference between Jetpac and Lunar Jet Man. They may have kept the same character, but he controls differently and the gameplay is completely different.
Instead of moving through an isometric maze slashing enemies with a sword, the explorer needs to find weapons, the first one he has is a slingshot (this needs to be picked up in the first room). He mainly navigates by leaping about like a frog which is a bit weird.
You can press a button to leap a direction, but there is an automatic jump he does when reaching the end of a platform which is mainly used for jumping down to a lower ledge.
The Underurlde is a strange place, partially looking like a mansion, and containing caves. You will be jumping over, and on, bookshelves, plant pots, paintings and more. In the caverns you will be landing on rocks, abseiling from the ceiling, and riding bubbles back up.
Just like many of these games, the number of enemies is ridiculous. It's even challenging to play without the enemies (in the Rare Replay collection, there is a section in the options marked Cheats). The enemies don't damage you directly, but knock you back. It's the fall damage from height that kills you. When it's a precision platformer or you are trying to abseil, then this is a huge problem because you are gonna be knocked to your death by the enemies.
Another aspect that makes the platforming difficult is that you often have to jump between screens. Often the platforms are positioned where you think they will be; for example they might naturally form a stair-like pattern, but sometimes it can be a bit trial and error.
When you get to the caverns with vertical drops, leaping forward automatically attaches you to a rope in the middle of the screen. You can swing back and forth to move horizontally, or lower the rope all the way to the bottom. If you get touched, then you fall off. I think it is random, but part of the ceiling can break and so you will get crushed by a falling rock instead.
If you collect the treasure, then you get some invulnerability to save you from falls.
The rooms look very similar and the maze structure is hard to navigate and remember where you have been. The general idea is that you need to find a weapon that can kill the next guardian that is blocking your path. But it takes several screens and lots of exploration to find the guardian or new weapon. Finding the weapon first saves you on backtracking at least.
I managed to get to the end using the no enemy cheat, and lots of rewinds to get through the tricky platforming. I got a completion of 34% which might be tracking treasure and explored screens. It felt like a long game too, so no idea how anyone would have completed this legitimately.
When it's hard to find where to go, and navigating each screen's platforming challenge is difficult; it's not exactly a fun game. Then throw in enemies, and you have an insanely frustrating experience.