Access: There are a variety of ways to get to this ballpark from Central Tokyo...and they're all a pain. 

Here's the easiest way:  Make your way to JR Ikebukuro Station. From JR Ikebukuro Station take a short walk while inside the building to the Seibu Line (a private railway) at Seibu Ikebukuro Station. 

From Seibu Ikebukuro Station you want to end up at Seibu Kyujo-mae, the stadium station. To do so, depart Seibu Ikebukuro Station on the Seibu Ikebukuro Line toward Nishi-Tokorozawa. 

Get off at Nishi-Tokorozawa (not be be confused with Tokorozawa).

At Nishi-Tokorozawa transfer to the Seibu Sayama Line and its train to Seibu Kyujo-mae, the stadium station.