Castlevania and Contra are two of Konami's biggest side-scrolling platform franchises. They reference each other from time to time, and are sometimes bundled together side by side.
In Contra: Hard Corps, in stage 3 before destroying the first door, climb the wall and you will reach a hidden section where you will meet a mysterious character. He will ask if you want to do money in the Arena. Respond "YES" and you will enter a room where you'll be tested with a series of miniboss fights.