This introduction is based on the Shulchan Aruch: Shin Mem Hey: Sif Beis (345:2).
A Tel HaMislaket is a hillside which has a steep slope and can be used as a wall. This particular slope is based on the Talmudic definition for a Tel HaMislaket of “within a distance of four Amos [cubits] the rise of the hill should be 10 Tefachim (hand breaths).
10 Tefachim (hand breaths) is approximately 40 inches high (There are differing opinions on exactly how large a Tefach (hand breath) is.
4 Amos (cubits) is between 6 and 8 feet. (There are differing opinions on exactly how large a Amah (cubit) is.
There is a difference of opinions (of early commentators) as to whether the four Amos we are referring to is the hypotenuse, the actual slope, or a distance measured along a flat ground which is under the slope. The difference between these two opinions is a slope of 24.6 degrees or 22.4 degrees. Common practice is to use the steeper slope of 24.6 degrees.
In order to use a Tel HaMislaket as part of an Eruv, one has to verify the slope and height of the hillside approximately every 10 feet (horizontally). For example, if you want to use 100 feet of hillside as a wall, you would need to verify the measurements approximately every 10 feet. Consult a Posek (Jewish Legal Expert).
One area of confusion is how to connect to a Tel HaMislaket using a Tzuras HaPesach. Where do you put a side bar of a Tzuras HaPesach? Consult a Posek.
Another area with differing opinions is can you use a Tel HaMislaket if it is “stepped,” or not straight on the incline.