To be fair, humans (almost) never used the fort building ability of legions either.
I think it's going to be hard to use well. Having a small number of charges to build them with is unusual, once it's gone - it's gone. Your legion is now worse off than a brand new one. I tend to be very conservative stuff like that (such as using potions in RPGs) so might struggle to make myself use it. I'll be in fear of needing it more later. Although once legions are about to be upgraded I might see myself use it very liberally before it disappears.
Btw, do we know the bonus from being fortified? I think it's +4 but I can't remember exactly. If so then a legion in a fort on a hill has:
+5(legion) +4(fort) +4(fortified) +3(hill) = +16
against other swordsmen, which are the strongest unit of the era. From what people have been saying that is enough to one-hit-kill a unit. So the legion would be almost invincible. Waiting another era and bringing in pikemen (str 41) doesn't help that much, legions still have a +10. Knights come closer, but are still weaker. Unless you have a special UU, you have to wait until muskets to have a unit which can beat a well fortified legion. Large numbers of ranged units could eventually bring down and so could flanking/support bonuses, but only if you are completely passive in your defence.
The more I think about it, the more I realise that Rome is not a conquering civ as before. It's an expansion then turtle civ.
A rough strategy: Get the policy that gives cheaper settlers ASAP, spam those around you as fast as you can. After having researched the techs you need for your immediate economic development, get to iron working. Start pumping out legions as the land to settle disappears and your neighbours might start thinking aggressively. Now *do not invade others*, but turtle up and catch up on infrastructure. Building lots of settlers and legions probably means you have fewer districts than others so this will take some time. Luckily the baths will help with that, as does having more lucrative trade routes and not needing to build monuments. By the time you hit the renaissance and your legions are no longer enough to keep you safe, you should have caught up in the development of your cities AND you'll have more of them. You can now take whatever path you want to victory.