Banshees are one of the most controversial units in castle fight. Some think they're way too strong, while others think they're lots of counters for them. I'm going to shed some light and share my experience.
As a disclaimer before i say anything further: I'm talking about private -pr3 -lm games between good people. None of this applies in public games.
The First Banshee
The first banshee is the most important banshee you get and it might be the most important single building in the whole round. To build a banshee, it is imperative that you collect the first coin, which nets you an easy 11 gold. Bansheeing in -nc games isn't viable. It's important to note that the banshee is the single best unit you can build with one coin.
In fact, I'm going to say this now: In a 4v4 private game, there is NO good unit counter to an early banshee push. (ie. the first banshee you build).
Why is this?
In 4v4 private games, both teams build in one lane and sync their first units into combat based on build time, movement speed, and in what order you want units to arrive to battle. Usually you want your most durable unit(tank) to get to the fight first, because all 4 of your opponent's units will be focusing whatever unit engages them first. This allows for low hp but high damage units to survive and do their job (ie. archers, bloodthristers, druids). The possible tanks include: banshees, defenders, kodo windigo). Usually whichever tank falls first loses the push for its team. This is the scenario for a starting push in castle fight.
Ok, this next part gets a little difficult to understand. The problem here is that no tank can outlast a banshee under normal circumstances. Why? When most teams see an Undead worker picked on the other team, they intelligently assume banshees will be built. Then, they assign one person to make a chaos-damage unit so their push consists of 3 regular-damage-type units + one chaos damage unit. The other push will consist of probably 4 regular-damage-type units. Your team will be doing 25% damage with 3 units and 100% with one unit to the banshee tank. Your opponent will be doing 70% to 175% to your team's tank. Even though banshees probably have much less hp than your tank, your team’s tank will almost definitely fall first. In addition banshees are usually paired with farrie dragons will heals banshees for almost 4x effectiveness in comparison to a regular unit. The point: banshees seldom lose.
So what if your team gets 2 or 3 chaos damage units? Well, the first foreseeable problem is that you’re limiting the types of units you get and many chaos-damage units aren’t especially good early pushers. Also, if your opponent’s team can simply sync so another unit is in the front to tank or better yet, build another undead unit that’s not banshees.
The most popular choices for countering banshees early game and the reasons they don't work include:
Bloodfiend: Unless you get lucky, this unit isn't very durable.
Zombies: These things don’t' have very good stats and again aren't very durable.
Frost dragons: Very hard to sync, because they are so fast and fly. Also, they're not very durable especially vs banshees. On an unrelated note, banshees actually beat these 1v1.
Naga siren: You have to save for an insanely long time and you will have lost the first push by the time you get these plus you'll be behind a lot on income.
Druid: Theoretically this COULD work, but druids extremely flimsy as well and if your team has banshees, you’re probably better off with a fairy dragon.
In my opinion, the best counter to banshees is super early building. If your opponent is making banshees, they will probably wait and sync on the banshee. If you get 2 waves pushing together before their first wave can come out, you will probably win the push. However, your units won’t be the best possible early game units (all under 255 gold or 275 if your unit has an upgrade like defenders). If the other team is extremely quick, they can adapt and counter your strat with quicker buildings.
During the mid game, units with chaos do pretty well, because there are no (or less) complex tanking mechanics going on. Naga sirens especially work wonders versus banshee. Also, tentacles become a viable option, because one group of tents can kill a
Remember that to win the game, you’re not trying to counter banshees; banshees aren’t the only units your opponent is making.
You must counter the combo of units and specials your opponent’s team is making. When the late game comes around, another strength of banshees is shown: They are highly resistant to spells and can be healed with extreme efficiency by wizards and glades. Late game is all about large presences on the battle field. Many games see 30 units on each side battling it out with reinforcement rates almost equaling death rates. This is the reason why long range siege units are effective late game (They build up because they never get hit by conventional attacks and don’t die). Banshees are a key part of this equation, because aoe spells are not very effective against banshees. These include: frost nova, chain lighting, warlock explosion, tidal wave, starfall. Wizards heal all the damage from these aoe spells very well and in fact, in many games, late game pushes consist of almost entire banshees and wizards.
Ironically, banshees plus wizards counter chain lighting from wizards, frost nova from ice queens, and the explosion from warlocks. I would go so far as to say they counter these units even though the units all have chaos damage. (Actually banshees definitely are good vs warlocks because warlocks don’t even attack enough with their regular attack for it to really matter.)
Units with regular chaos also don’t work very well, because they hardly ever attack banshees (same principle with early game where all units focus one unit on the other team). In addition, generally late game is all about aoe spells and specials(which are spells). Units are too busy dying to aoe spells to actually be effective in battle. The only things that are effective versus(notice how I didn’t save counters to) late game banshee are units with chaos splash. These units include azure dragons, red dragons, incubus and infernal. All of these are effective against both regular units and banshees.
A fun analogy
A good analogy from Warcraft melee concerning with countering a strategy: Nothing the orc army has really counters mass druids of the talon.
Theoretically, raiders counter dotts, because they do extra damage to them and take less damage from them.
Theoretically, spirit walkers counter dotts, because they can dispel all of the dotts’ spells and also redistribute the damage from dott focus fire.
Theoretically, the tauren chieftain counters dotts, because it does aoe damage to lots of units
Theoretically, the blademaster counters dotts, because it rips up low hp units.
So many newbies get all 4 of these things and fight a dott army and lose horribly. Why?
They’re too worried about countering dotts and not the whole overall strategy. The elf can position his army so none of your units can hit dotts. It is extremely hard to maneuver around 3 heroes, 2 quillbeasts, and a pocket factory that your opponent gets. Position is often more important than army size or composition. Therefore, raiders will have a hard time actually attacking dotts. In addition, although dotts do reduced damage all the units and heroes I named, the elf heroes will also be throwing out extremely high dps. Although spirit walkers can dispel, they’ll never have enough mana to dispel every single cyclone, because you can never have more walkers than they have dotts. The TC is negated by mana burn and scrolls of healing. The blade is kept constantly in the air. Although you can dispel the cyclone, it is very hard to react immediately, so the blade will always be in the air for a few seconds.
You’ll be surprised to find out that orcs actually do get this combo of units versus mass dotts. The key is timing. You can harass the night elf and attack him during very small windows where the elf is vulnerable. Also, you must avoid fights, many times trading tps. While doing this, you must slowly level up the tauren chieftain so that his shockwave will overwhelm the number of scrolls of healing the elf has. During a fight you must ensnare the dh out of range of the TC and keep the dh away from the TC as much as possible. In addition, you must alternate between focus firing heroes and dotts, because if you mass focus heroes, your opponent will just try to soak the damage will doing constant dps to your army, but if you mass focus units, the elf heroes won’t die and level up extremely fast, overwhelming you. None of this is very intuitive, but this is how it works.
On a completely different note, banshees counter the early orb of lighting + scroll of stone combo strategy after a strike, which often forces a team to use 2 strikes one immediately after the other, because they don’t die to orb of lighting.
My point? Banshees are strong at all points of the game.