I discovered a great use for Barricades, while playing C3C Napoleonics scenario. Previously, I used them to narrow the front, so the AI would use just one approach route. But the better use is to take advantage of their ZOC effect. Set them one tile apart, preferably on good defensive terrain. Pack them with whatever passes for inf and arty: the inf to defend them (with arty help). As the attacker comes adjacent, welcome him with your arty. As he moves between them, some get nailed by the ZOC. Give him your arty's regards once again. Some of the stack can then be attacked by the inf in the Barricade, thereby gaining experience, and slowly healing afterwards. Mop up with Cav from the city, retreating afterwards back into yon city w/barracks. Sometimes, the attacker takes such a beating from the ZOC's, that after being shelled again, he heads home to get healed, and passes through the shelling again! If you're light on units, you can wait for this to happen to a select few units, let them escape, then shell and fight the remaining units. The strongest units will be the ones which got shelled first, so you'll be fighting his weaker units. Don't forget: Armies have Zoc's, too! The opportunities add up. Use an Army to clear out some of his tougher units, then make sure it's positioned along the retreat path, then shell the remainder. I was outnumbered 3-1 by Russian Guard units, but I wiped out about 30 or so over 5-6 turns, without a single loss. I repeated this as I advanced into Russia, taking towns, building a line of Barricades, and crushing the counter-attacks. It gave me just enough time to bring up garrisons for the towns I'd taken, and to suppress resistance.
This works nicely with a small force, too. Send a group with workers into enemy territory, and park them on a hill next to his approach-route road. As he sends his force down the road towards your territory, a few will get nailed as they pass, weakening them just enough to defeat their assault. It's a great way to develop elite units in your territory. Find a spot that AI will attack, and rotate defenders. The weakened attackers lose the fight, and you bring up fresh defenders. This is called rope-a-dope. You can do this all day. Here's the funny thing: You can set this up easily during a RoP agreement! When you're ready to fight, don't renew the RoP, and refuse to leave enemy territory. If you're worried about rep, just build the barricade, and move a stack into there later after DoW (a little tougher trick).
Regarding fast units: they don't withdraw from combat very well, if the tile behind them is enemy-occupied, so use your roads to attack from opposite directions, and you can wipe out that pesky cav. If they're in a line or in several adjacent tiles, attack the rearmost ones first, then the others will fight to the death.
This works on a larger scale, too. I was the French fighting the Spanish at the border. The Spanish had some 25+ Cav filtering through the hills, the way the AI is wont to do, if you block the roaded passes. With inf to reject them in the Pyrenees, I was able to get behind them, and take the town. Using the roads, I formed a classic pocket. It was aesthetically very pleasing. First, the AI didn't know which way to go, so the few attacks on my inf weren't followed through on, as some cav headed back towards the town (and towards some bait weak units that they couldn't reach in one move). When I attacked the mass in the hills, there were no retreats from my inf. I hit the full-strength units with my cav, and hit the rest with inf. They died where they stood. I did something similar against the French as the Austrians, on a smaller scale.
Aiming Arty: you can hit every time you fire with these things, regardless of type (even catapults). When you aim, the left edge has to be exactly between the two borders of the targeted tile. The right edge has to be just intersecting the right inner border. It helps to have the map centered on the firing unit. The trick is not to move the cursor when clicking, or while waiting for the animation to finish. Artillery is the god of war. I'm a big arty fan. I have tested this again and again. I have literally fired tens of thousands of shots in Civ games. Take your time aiming, and it works.
Ships as Arty: if the enemy nation is like North Africa, and reinforcing units march along a coastal road, then park ships that bombard along that coast. Dromonds are the best for this. You can weaken units as they move up to the front. You can slow the reinforcement rate, if they stop at a town to heal, but more likely, they continue on into combat in a weakened state. It's very rewarding, as you send your ships along the coast seeking victims. For added fun, in a time-critical situation, you can follow them along the coast, blasting them all the while. Very satisfying....