Panegates are apertures which resemble a barred window, but they show and open to distant places and to parallel universes. As well as the bars, panegates have a transparent barrier which is not glass, even Ludwig was unable to break it. Each has a panel with many buttons. If the correct sequence of button-presses is known, the bars will retract and the transparent barrier no longer stops the passage of solid objects, although it appears to keep out opaque air or liquid, as seen here. A closed panegate is invisible at its destination, but still permits viewing from the gate side.
Opening a panegate is a dangerous operation. The control panels have a defensive mechanism which is likely to activate if the operator gets the code wrong. Perhaps it always goes off at a certain point in some sequences, requiring the operator to remove hir hands. Panegates which link to other parts of the Mansionverse have especially complex and dangerous controls, perhaps due to the war with the Sneeches. Panegates which open to parallel universes have simpler controls, but few if any are entirely without danger. This is not a hard rule, "a couple of the very simplest ones" open to the Mansionverse, but they open close to the Mansion of E itself; to the surrounding forest and to a cave in the nearby mountains (ref).
Lemuel (the 12th Earl of E) was found dead in front of a panegate with a horrible expression on his face. Whether some sight shocked him or he was killed attempting to open the gate is not known. It is also possible he entered the gate, was poisoned on the other side, and just made it back before he died. In his lifetime he entered more panegates than any other mansion resident on record, perhaps more than any other human being.
In the Mansion of E, no panegates link to the other continent of that world, indicating it was probably Sneech territory, the Mansion of E having been an Ettin construction. It's safe to say the Mansion's panegates were made by Ettins, no other species inhabiting the Mansion attained sufficient development to do so.