Exiting the camp or how to determine Porta praetoria

[D B Campbell]

first- besides the fact that porta praetoria (semper hostem spectare debet) is facing the principia, what is another specific characteristic of this gate. Who was exiting through this gate? Any other important feature of this gate in terms of organization?

Hyginus (de munit. castr. 56, which you quote) says that the porta praetoria should always face the enemy. (But of course it's called the porta praetoria because it leads up to the praetorium; Hyg., de munit. castr. 14.) I expect the matter of who used this gate as an exit depended upon local circumstances.

second - is it possible that (why not) all four gates were used when the whole legion exited the camp?

I don't see why not. The large number of camp gates (usually four but sometimes six, and sometimes more) is usually explained as providing additional exits.

I am trying to make the difference between two gates of different dimensions without the knowledge od principia position, lol.

The only way to identify the porta praetoria in a temporary camp is to identify the portae principales (which is not always possible) and hope that your camp is tertiate (i.e., divided into thirds, of which the front third, as defined by the via principalis, will be the praetentura). A camp like Rey Cross, with three gates in each side, does not allow this kind of analysis.

Hope some of this helps!