More to add: The rhine and the danube are your best friend. Without them keeping Western Europe civillised would be impossible, instead of merely extremely difficult. There are just over half a dozen bridging and fording points along their lenth. Build your army towards having a small border army on each bridge (Im working with 4 Limitanei and 4 Archers, though some are underutilised and Ill be disbanding them as the horde threat recedes), and then build two field armies with all the good stuff and sit them centrally to reinforce any bridge that comes under threat. Get them led by a general with good movement bonuses. With highways built ( midgame priority) youll be able to move along whole frontier in one turn. Best way to discourage hoardes whilst keeping your army bills down. Then your down to some good old fashioned bridge massacres!

In my game Ive so far annialated the Goths and the Vandals (theyre still around, just not worth anything). The Vandals were seiging the Frankish capital, and I was considering my options when the Vandals forced my hand by suddenly crossing into Roman land via a ford just north of the Alps that I hadnt seen and thus hadnt defended. Cue panic! Anyway, the interloepers were driven out, but better still was the night attacks led by that treacherous cur Nero to lift the seige of the Frankish capital by the Vandal stacks. I could take each stack singly because of night attacks and thus managed to beat them all with a scratched together army. The real benefit of lifting the seige on the Franks was stopping them from "hoarding" and thus making the Rhine to hot to hold. Nero and Spuris Flavius ( Acquinium) are extremely useful for cutting down the hordes to manageable numbers with their night attack ability.

Dealing with horse archers is a pain, but I favour heavy cavalry over archers against them. Youll take losses using your archers but your generals ( esp when retrained in Romes foundry) will be practically invulnerable to horse archer fire. Takes some micro management (and a few generals on hand) to "herd" horse archers against a red line and massacre them but its the most cost effective way to do it ( gens regenerate, archers do not). It also helps your gens develop good traits/stats from all that killing.

Right now, there are only two hoards still active - the Huns whove taken a few beatings on their way to me I think, and the Samartians who are probing to find an unguarded bridge. This is something you can manipulate - when the Vandals crossed into the Alps the Samartians were queing up to follow them across this ford. By this time I managed to get an army to the ford, and pulled another army off the bridge north of Acquinium - Samartians turned straight around and made a beeline for the unguarded bridge, giving me the turn I needed to beat the Vandals and I was still able to garrison the bridge again before the Samartians arrived. The hordes really dont like guarded bridges.