Straight from the Mariners website:
The Bullpen Pub provides a sports bar experience with the fun and excitement of a ballpark atmosphere. Fans can peek into the bullpen and watch the manually operated scoreboard from behind. You can access the pub via the Bullpen Market through the Center Field Gate (open three hours prior to game time).
I didn't find anything requiring extra admission, like the Hit it Here Terrace, so I'm assuming you can just hang out there. Someone please correct me if I'm wrong
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Have a great day!!
~~Missy~~