The Castle 3 defaults to auto lipo cutoff. You didn't mention what battery you're using. But 7 cell NIMH batteries look like a 3 cell lipo to the ESC when it's in this mode, but the voltage is just barely over the cutoff, so as soon as you run it, the low voltage cutoff kicks in.

You might be able to verify this when you turn the ESC on. When you first turn it on, it makes some noises, then it will do either 2 or 3 beeps, then another set of noises. The 2 or 3 beeps is the number of lipo cells it detected (or thought it detected).

You can turn off the LVC by following the steps in the manual.

If you're already running a lipo battery, we have some other problem.