Ranarama, by Steve Turner of Graftgold, originated on the ZX Spectrum in 1987, and was later converted to the Atari ST, Amiga and Amstrad CPC.
In my opinion, the Atari ST version is the best, because it has the best graphics and a nice, speedy feel to it. The Spectrum original is great, but the 16-bit versions have better use of colour and higher resolution graphics.
In Ranarama you play as a sorcerer’s apprentice, Mervyn, whose botched spell has turned him into a frog. Gameplay is viewed from an overhead perspective, and the aim is to basically hunt down a pack of evil wizards, over eight different levels of a large dungeon. Rooms only become visible once explored, and there are lots of invisible doors to contend with too.
The deeper levels have tougher monsters, and are not immediately survivable, so the idea is to build up your offensive and defensive spells before exploring them.
More: Ranarama on Wikipedia