Lyric discussion by mzwtjp 

Cover art for The Great Gates of Kiev lyrics by Emerson, Lake & Palmer

Obviously, this is a metaphor of sexual intercourse and its consequence. Something comes out of love's tower. Fluid stirrs and be poured into a gap, where old dried sun resides. Then, pushed out of the gate, with tide, and you are new born. By giving life to new born, you will live forever, even if your incarnation of body dies.

My Interpretation

@mzwtjp another line also, when it says "and in pain there must be gain, new life" likely a reference to the pain of child birth, and the creation of a new life