All of the following tips and tricks were provided by local awesomenaut TheCutestFaye! Huge thank you
Easy to get to secrets and levels.
On any level that says the dead are restless:
If you blow up the grave marked ASH you get a shotgun
If you blow up the big tomb with the crown there will be an exit that leads to the secret
Castlevania level. You unlock a character here.
On any level that says I hear Snakes
If you go to the bottom of the snake pit and blow up the center block you will get the pickaxe.
It allows you to dig up gems and gold.
On any level where you see the big pink blob on the wall:
If you take your dog and jump on the blob you will stick. Stay there and don't move and a worm will eat you and take you to the worm level. You unlock super meat boy there.
On the level of the Ice caves that says It feels like the fourth of July:
There will be an alien spaceship on the side of the level. Go to the top and there will be an exit
to the alien mothership secret level. Not sure if it unlocks a character.
Now for the hard one. This is a many step process.
HOW TO GET TO HELL:
Started in the mines. You will find a key and chest with a gold lock. Unlock it to get the ughet eye. This allows you to see gems, but more importantly when you get to the Jungle it will show you where the black market is.
In the jungle you must walk back and forth on all levels to make sure you don't miss it but the Ughet eye will start blinking and making beeps. This tells you the black market is nearby. It will be in the ground and usually takes at least two bombs to uncover the entrance. And you also need to have 50,000 gold at least
In the black market walk all the way to the left on the second level of the shop and there will be a guy who sells the Ankh for 50,000. Buy it. Get what other supplies you can and leave.
The next part requires you to kill yourself on a certain level of the Ice Caves. There will be a level in the ice caves with a huge statue of a blue head. Kill yourself on that level, it doesn't matter how. You will respawn in that head and a door will open. Wait like 3 seconds for the item you get to actually fall into your hands, the item is called the headgit. it is like a gold helmet. Then walk through the door.
Then when you get to the temple you have to find Annubis. The dog god. He shoots purple bolts that kill you instantly. Best way to kill him is two bombs that have the sticky glue on them from the spiders. And when he dies he drops a staff. Pick it up...I wouldn't suggest using it. It can kill you really easily.
Take the staff to the next level of the temple, while on that level you will see a door that is gold and has a key hole in it. The staff unlocks this door. Go in
This is the city of gold. While here you will see a small room with a book in it. It is the necronomican. I don't know how to spell that sorry. You have to bomb your way in get the book and kill the guy protecting it. Then you can leave that level.
You should be to the final boss of the normal game by now, Ughet, a big gold head lol. Walk along the floor where he is and at some point your book will start chomping. When that happens you know that the door to hell is at the bottom of that point. Easiest way is to get the boss over to one side, bomb your way down. Get the boss to chase you and fall in the lava, and you ride him down to the door. It is complicated and takes a million tries. But he has to fall in front of the door for it to open.
Then you go in and that is hell. Defeat the boss there and get the second ending. Make sure you have bombs because if you kill him there are a bunch of rubies under his throne.
You can sacrifice the idol, and the pug, and dead enemies, or stunned ones if you are brave enough at the alters to kahli. They are the bloody rock alters. The first time she gives you a random item, then when you sacrifice enough she will give you the Kappala. It looks like a skull cup, and when you kill enemies you will collect their blood in it and when it fills you get health. You can also continue to sacrifice and get blood that way as well