What is the reason I am in the Skull Castle? Everyone confused me at my last post. And where is the underground place and how do I enter?

You are in Skull Castle to get the Lich's Staff.
You need the staff to open the sphynx pyramid which will allow you into the final area of the game.

In Skull castle you need to locate the tilting bridge.
Once you fall in the water there are 4 exits.
The north west exit will take you to the room which has the key that allows you into the room to TALK to the Lich. To get his staff you have to find a relic for him. The lich will give you a key that opens a gate to the lower levels of the castle. You can find this gate by (from the entrance) going to your right and heading down the stairs.

There are many other things to do in Skull Castle too.

Wyv

You have quite a few things to do in Skull castle actually. Many side quests take place here including killing the Vampire Madda Mebig. Your main objective though is to get the Staff of Death from the Liche. The staff is needed to get into the Temple of Anephas which will let you into the next area.