Sea Gate offers an interesting insight into the area's past. Look up at the walls and try to imagine the scenes that occurred around it throughout its history.
You're spoiled for sightseeing choice when spending time in Zadar Old Town. Sea Gate is an ideal place to kick off here, but there are lots of other fine places to consider. Forum, St. Donat's Church, and Land Gate are not far from the historic structure and will round out your day nicely.