Venus Lighthouse is the elemental home of the Earth, or Venus, power. It rests on the Gondowan Continent's northeastern edge, but unlike Mercury, the door isn't so easy to find.
As you go down the stairs from where you left off at Babi lighthouse, run south and walk down a set of stairs, then through a door. Here, you will see two pillars with yellow tops. Move them both so that they are one space from the right wall, and touching north-south. Go back up and walk onto them, then jump across and go through the door to the south. In here, jump down and to the bottom-left of the room, going through the door.
In the next room, you'll see more pillars with yellow tops. Walk down the stairs and Move the left one down once, then as far left as possible. With the right one, Move it all the way to the bottom, then as far left as possible. Climb back up the stairs and jump across the pillars to the ground on the left, heading through the door at the bottom of the room.
This room contains some vines and small pillars. Walk down the stairs and Move the left one all the way left, then Move the right one three spaces left. Walk back up the stairs and jump across the pillars to the large vine overgrowth. You're going to have to Whirlwind the left and right side of this, revealing yet another pillar. Move it over to the left once, then down off the platform. Once you have done so, hop back and go down the stairs again. Move the new pillar to the end of its groove, then move the original right pillar as far right as you can. Follow the new path opened up by this to the door, which you will have to Whirlwind to get through. There is nothing in the next room, so just follow the path down to the next door.
The room you are now in has three carry stones, as well as balanced weight scales. Move the far left stone over one, then Carry it over the scale in front of it. You'll see the scale on this side sink, and the scale the over side rise. Move the middle pillar right once, and do the same thing. You can now jump across to the left side of the room, which leads to a door at the south.
This room has many platforms leading to two sides. Take the left route to enter a room with a statue. Use Reveal to expose a treasure chest where the statue is, containing Oracle's Robe. Go back and head down the right path to the next room. This one contains 5 statues of 5 different colors, with marks on the floor for each. These are no ordinary statues; you can actually use Mind Read on them for hints as to where they are placed. The order should look like this:
Once this is done, the door to the right will open. In the next room is a set of stairs that leads to the REAL Venus Lighthouse. There are no monsters here, so you're all clear to go ahead without hesitation.
Once you are in Venus Lighthouse, there are a few paths to choose from. The stairs on the far left will take you to the room with the electrical bolts from Your first visit. If you need to leave the tower for any reason (you shouldn't; there's a Psynergy stone right here to heal you up.) you can do so by moving the statue onto a button, disabling the electricity. Note that if you DO decide to exit this way, you won't be able to return through it and will have to do the Tunnel Ruins again.
Instead, go through the center door. It leads to a room with flowing sand, like that seen in the Lamakan desert, which mimics the waterfalls in Mercury Lighthouse. In here, head to the left and go in a door. You will see a Carry Block here; naturally, use Carry and slide it over one to the left, dropping it in a space. Jump across and head through the door. Ignore the stairs in front of you and walk north as far as possible. Step out into the flowing sand and walk right as quickly as possible. This is rather tricky, but try to step onto the narrow ground at the very edge of the sand as it falls into the gap. If you fall off, walk up the stairs in the room below and try again. Once you are through, go in the south door to get the Thunder Crown (a cursed equip), and then head back and up the stairs at the north.
This room is like many in Mercury lighthouse, with several "sandfalls". Walk north into the one farthest right to enter a new room. Head right until you see many platforms, and then jump across them south, taking the rightmost path. Once on solid ground again, head left and through a southern door. In here is a Statue and a piece of track, with a hole in the floor. Push the track into the hole and the statue will emit a light down the track, opening the door. Go in the newly opened door, and step on the button in this room. You will see that the sandfalls in the second room changed. Head left and go down the stairs. Now you are back in the second room. Go south and walk around to the right side to climb down the ladder. Move to the north, climb up the ladder, and walk to the right across the sandfall.
In here, head south and climb down a ladder. Move two pillars to the south and pass through a door that they were covering. Push the pillar all the way to the left, then go back and climb up the ladder. Jump across the path you just created and go up the stairs. This will take you back to the room with the platforms. Head south, then climb up the ladder and go up the stairs. Walk right through the small room into an area with another statue-track puzzle. There's more than one right way to do it, but one is to take the upside down L piece and put it in the northern right hole, the backwards L and slide it into the southern right hole, and the straight piece into the southern left hole. Once the door is open, go through. Jump across, walk through the sand, then head north and jump across again. Go into the south door, and walk north into the first sandfall to get a Dragon Scales from the treasure chest. Go back out and walk into the second sandfall to proceed.
Use Carry to drop the Carry Blocks onto the left weight. The platform the Carry Block lands on will sink, and the opposite one will rise; you only need to drop two, but there is a third in case you drop one off at the wrong place. Once across, go through the door. Stand on the button to open up the sand's flow again, then jump north and go down the stairs. You have now started the flow of sand in the leftmost side. Use its current to go south and then walk left onto ground. Go up the stairs, and in that room through the door on the right. This is a maze with a sand current flowing through it. There is a treasure chest in here containing the Gaia Blade, the best weapon for Isaac in the game. Make sure to grab it. Try to reach the right door by using the sand's current (after getting the sword, of course.) and go through. In here, just walk over and go into another door further right.
This room contains a more complex statue-track maze. Ignore the left path entirely, and very carefully move your pieces into place on the right side. First, put the vertical straight piece into the rightmost southern hole. Then, put the backwards and upside down L piece into the hole that lets you choose between two paths; this should divert the light to the left. Now, slide in the correctly facing L piece at the hole where the left and right paths meet. Finally, move the upside down L piece into the hole directly above that. This should create a complete track, and the light ball will race down it and unlock the door. Once it's open, go through and stand on a button. This will start the third sandfall flowing again. Jump across to the north and walk down the stairs that are on this side. In here, head south and you will come out from the center sandfall. walk onto the solid platform, as far north as possible, then quickly run across to the right side. Once across, ascend the stairs in both this and the next room. Heal up, grab the Psynergy Stone, and save. This is the point of no return.
IF YOU HAVE ANY DESIRE AT ALL TO COMPLETE THE SIDE QUESTS, DO NOT GO FURTHER!