head to head
Neither is simply harder - they are hard in different places. Centurion: Defender of Rome demands more variance; Portal demands more execution.

Optimization · Pure Optimization
Managing provinces, taxes and legions as a solvable empire-building system.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Centurion: Defender of Rome leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Centurion: Defender of Rome
Issuing orders and chariot minigames need little dexterity
Portal
Portal placement timing modest.
Centurion: Defender of Rome leans a little harder on reads, though the gap is small.
Centurion: Defender of Rome
AI rivals follow fixed diplomatic and military logic
Portal
No adversary modeling player.
Centurion: Defender of Rome leans clearly harder on variance.
Centurion: Defender of Rome
Battle outcomes and rival behaviour carry some hidden randomness
Portal
Deterministic chambers; no randomness.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Centurion: Defender of Rome
Provincial economy, taxation and legion deployment are solvable systems
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.