First and foremost: thanks go to MercilessMerc for helping with the location of the 2nd and 3rd key, I only found the 1st one by myself.

I'll provide increasingly helpful hints about the locations so you can just use the general area instead of the images.

Matt's Key 1 (need Ruth's Key from House of Ruth)

Area: Castle Entrance

Tip: The entrance is near the save point that is in a room filled with spikes, nearby you have the dark rooms that lead to the Torture Chambers.

Guide: Right of the dark rooms that lead to the Torture Chamber area you enter a door that leads to this place:

There is a secret entrance on the left wall which will take you to this room:


Matt's Key 2

Area: Torture Chambers

Tip: After the 3rd savepoint there is a sawblade that may hold a secret in its path.

Guide: Just keep going after the 3rd savepoint until you reach this room:

As you can see the floor is not solid, you then move right and fall into a room with spikes and the key:

This jump is possible and will save you a trip back through one particular jump that is harder than this one. When you enter the door you will have to quickly jump to the side as the door teleports you over spikes.

Matt's Key 3

Area: Windy Ruins

Tip: After the savepoint that is after the warp point in this area, it is a a breakable block so you just have to look really well.

Guide: Continuing through the savepoint you'll reach this place:

The place I'm standing was a breakable block, you just enter and complete the platforming challenge inside.

After collecting all 3 keys you can enter the Dev Room which is located west of the Clock Tower. It's just a matter of jumping from the Clock Tower top to the left and there'll be a house with the door open since you have the 3 keys, just enter and enjoy.

Top of Clock Tower:


Dev House: