head to head
Neither is simply harder - they are hard in different places. Mundaun demands more variance; Portal demands more execution.

Optimization · Pure Optimization
An alpine horror tale testing puzzle deduction and exploration under light dread.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Mundaun leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Mundaun
Sparse hostile encounters need only basic evasion
Portal
Portal placement timing modest.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Mundaun
The valley's horrors follow folk-tale scripts
Portal
No adversary modeling player.
Mundaun leans clearly harder on variance.
Mundaun
Alpine secrets are hidden until uncovered
Portal
Deterministic chambers; no randomness.
Portal leans clearly harder on optimization.
Mundaun
Hand-crafted puzzles and routes have fixed solutions
Portal
Pure puzzle-solving optimization.
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.