The brick observation tower in Volunteer Park, build as a shell over a water tower, is now clearly the highest ground on the largely flat summit area of Capitol Hill in Seattle. A staircase leads to an observation area on top, and boulderers enjoy climbing on the lower sides of the brick walls.
According to the 5-foot contours on the King County GIS iMap website, the highest point on Capitol Hill is just south of Volunteer Park and the observation tower. There are two square-shaped 455-foot contours on the west side of the two blocks bounded by 14th, 15th, Prospect, and Aloha. These areas are almost entirely on private property (residences).
That said, all the land in this vicinity is altered by man over the course of the past century or more, so there is no natural ground left. And the area is largely flat. It is hard to deny as ascent of Capitol Hill to anyone who has been to the tower. When walking south on 14th Avenue and east on Ward, there does not appear to be any higher ground than the earth built up around the tower base.