Impressions last, so the imperfect dominates.
Because seeming is a state rather than an event, il semblait is by far the more common past: « il semblait inquiet » — he seemed worried. You are describing an atmosphere, and atmospheres take the imperfect.
Il a semblé exists but is rare and marks a brief impression at a specific moment: « il a semblé hésiter » — he seemed to hesitate, just then. If in doubt at this level, choose the imperfect.
💡 Focus at this level: Past — how things seemed
1. Default to « il semblait ». States take the imperfect.
2. « Il a semblé » is momentary. Rare, and deliberate.
3. Pair with descriptions. « Tout semblait calme. »
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