Yer take the heat oot of air or the ground and put it into a sealed transfer system, using electric to pressurise sealed air in order to transfer said heat. Tis the same way a fridge or air con works. Only difference is yer want to circulate the heat round yer castle. Air con pressurises and lets go of pressure, to transfer heat, in a fixed system.
Its the pressurising of the sealed air that transfers the heat, into hotterer heat, thats hot enuff for you to feel warm, is the magic trick. Yer not generating the heat yerself like when yer burning somefink. HSP works by transfering heat. Yer only pay the cost of transfer. Hence why its considered so efficient when compared to burning gas.