Doomsday Castle

The castle comprises of single-screen rooms, each with multiple exits which begin sealed, and are also sealed when used (so as to prevent backtracking). Your gun's missiles are only effective in destroying these walls whilst charged with theta radiation. Thus, you will have to shoot Urks (8 for a full recharge) along the way to keep it active.

Each room is followed by a passage, which has time-locked doors and thus involves surviving against further tough enemies - Neucleods breed rapidly to create large bolts of energy, while Orphacs bounce off walls to provide primitive security. You have 1000 points of energy, which are lost with contact with these roaming creatures.