Where do you place the borders of your empire?

[Centurio Nixalsverdrus]

I like rivers, mountains and seas as borders. I only build forts at choke points when I don't plan to invade the other side. When there's still a war in the process with the possibilty of territorial gain I think it's better not to bother with defense logistics and instead fight the enemy in the open whenever it's possible.

Currently my empire has three "limites", a Thraikian Limes (I really don't want eastern europe, but Carrodunum might be convenient to swallow, we'll see), a Kimmerian Limes (no I don't want the steppe, not at all), and a Kaukasian Limes (same reason).

I don't follow the province boundaries always, instead I defend at choke points such as fords and passes, and in the Kaukasus area, a great part of my defence line is in Sarmatian or Eleutheroi territory, whereas little parts of my lands are only guarded by watchtowers.

The fort-garrisons consist of six units each, which are as follows:

Thraikian Limes:
1x Illyrioi Hippeis
2x Keltohellenikoi
1x Autokratorikoi Hoplitai Panoploi (I invented them, look like H. Indohellenikoi), plan to replace with Triballoi
2x Iaosatae

Kimmerian and Kaukasian Limes:
2x Skuda Baxy (sorry, the rider variant)
2x A. Hoplitai Panoploi
2x Skythian Archers

For when there is a war going on, and I face frequent invasions, I prefer a kind of "adaptive hinterland defense". My empire is divided into themai, military districts, that have a force of regionals in their capital (1x cavalry, 4x infantry, 2x missile). At unruly borders, these force is augmented to 2x cavaly, 4x infantry, 8x A. Hoplitai Panoploi, 4x missile and is often lead by a strategos.