I played a spell with my unit on Abandoned Hall. My opponent defies it and resolves. Does my unit still get the +1 from Abandoned Hall even if it was countered?

No, your unit does not receive the +1 Might from Abandoned Hall.

According to RiftJudge FAQ #10093 and Rule 412.1.b, a spell that is countered is not considered to have been played. Because the spell never finishes the resolution process, the "when you play a spell" trigger condition on Abandoned Hall is never met.

As clarified in RiftJudge FAQ #10217, abilities like the one on Abandoned Hall trigger only after a spell has successfully completed its resolution. If the spell is countered, it is cleared from the chain without resolving, and therefore the ability does not trigger.