Buy some helpful items in Aljiba and get a new crystal, then head to Rover Mansion.
In Aljiba, you can buy Garlic and Laurels. The Garlic can be found if you go through the door near the sign at the entrance of town and throw Holy Water at the floor. Garlic can be used to get some free items, so you might want to get some.
To get Laurels, go to the east side of town and go up the stairs, then go into the door there and throw Holy Water at the floor. Laurels make you temporarily invincible, which is especially helpful if you want to cross a poison swamp. You don't need Laurels yet, but you will need them eventually, so you might want to buy some now.
Get the Blue Crystal
You won't be able to go to the next part without the Blue Crystal, so go to the upper part of town and find the grey man in the upper-left. Talk to him and he will give you a Blue Crystal in exchange for your White Crystal.
Go to Rover Mansion
There is no church in Aljiba, so there isn't really anything else to do in town. If you bought some Garlic, you should use the right-hand exit to leave Aljiba. You will be in Camilla Cemetery. Use a Garlic and an Gypsy will appear. Talk to him to get the Silver Dagger. It costs one heart per use.
Then go to the left to go through Aljiba. You will reach the Aljiba Woods. Keep going to the left until you find some stairs. Go down those stairs, then go to the right.
You'll be in a cave. Go to the right and jump onto the lower platform. If you jump down to the ground to the right, you can go to the right to avoid the Skeletons up above. Keep going to the right. After some stairs, you will see a block to the right. Jump onto it, then jump to the upper level. Go to the right, then jump down below. Go to the right again.
You will be at Yuba Lake. Equip the Blue Crystal (which you should have gotten in Aljiba earlier) and kneel. After a few moments, a path below the lake will appear. Jump down, and don't go down the stairs. Just go to the right from there. You will arrive at Rover Mansion.