I want to make a dungeon grinder for a Wisp in a Bottle. Casters are giving me a problem, though. How can I make sure they don't spawn to specific areas? How do these "slopes" work?
I don't think there is a way to stop them from spawning. However, there is a way to make them easier to handle. Here is a step-by-step procedure.
1) Hollow out a large portion of the Dungeon, at least 100 tiles wide and 40 tiles high (make sure the backwalls are in back of every part of the section you hollow out). You will probably have to make the section in a tall part of the Dungeon.
2) Build a platform in such a way that you cannot see the bottom of the pit, but also so that mobs can still spawn on the floor of the pit.
3) Stand on the platform and barrage the floor below with Chlorophyte Bullets/Rockets/Terra Beams/whatever. All you have to worry about is the occasional Paladin.
As a plus, (which is what makes the casters easier) the casters will only be able to spawn on the floor or the platform. If you're on the platform, they'll only spawn on the platform (and vice versa for the floor), so they should be very easy targets if you're using, well, pretty much any weapon.
You can make a block sloped by hitting it with a hammer. I do not think that this affects monster spawns, however.
From the Wiki:
When the game decides to spawn a monster, it will first choose a random tile within a rectangle around the player. The maximum distance from the player it can choose from is 84 tiles to the left or right, and 47 tiles up or down. If the picked tile is solid block, it will try another one. If the picked tile has a wall that blocks monsters from spawning, it will also try another tile.
Once an empty tile is found, it moves down until it finds the ground. If there is enough space for a monster to be spawned at this location, this will be the tile it chooses. The exception to this is when the player is in an area where Harpies can be spawned. In this case the game does not attempt to find a ground tile.
The chosen tile is then checked to make sure it is not within the "safe" area around the player. Monsters cannot spawn less than 62 tiles away left or right and about 35 tiles above or below.
There is nothing in there that would lead me to believe that a monster cannot spawn on sloped ground, since for most purposes, sloping ground is purely aesthetic.