As sad and humble as I am, some of my earliest and fondest memories feature insisting to my mum that I wanted to climb up and toddle along this little mound of cobbles on the way into St Werburgh's Square and the rest of town. Ah...great fun!
Is it simply a left-over as part of the older street or is it something else all together?
Your best asking somebody with access to some old birkenhead maps to see what was there before that structure on top. That would be a good start to make I think. Derek has some old maps I think, he has helped me out before with my enquiries