At first glance, the Ottoman Empire's campaign in Empire: Total War might seem to be the most difficult. With poor lands, an awful cabinet, a very poor selection of early game units, and powerful enemies on nearly every side, the first few turns of the Ottoman Empire are probably the most challenging experience a player can have in the Grand Campaign. So here's a playthrough I've been doing. I'll keep the blog posted on experiences and what I could have done better. This is played on VH/H, with DLCs purchased.
Early Unit Overview
Generally, you want to build the very bare minimum of what you need to survive. Your early game roster SUCKS TREMENDOUSLY. They are, by a considerable margin, the worst units in Europe. Your line infantry are comparable to European militia! You'll disband them later anyway in favor of better troops, and every spare penny should be devoted to whipping the economy back in shape. That being said, here are your options...
These are the best equivalent to line infantry you can obtain in the early game. Isarelys are also some of the only units that can utilize bayonets and square formation. Compared to actual line infantry, however, Isarelys are awful, with all-round inferior stats and no rank fire ability. Palestinian Auxiliaries have better accuracy but not much else; in addition, they require a barracks in Palestine which is pricey, and cannot use bayonets or square formation, making them inferior in a melee.
Regardless of what is chosen, be prepared to support them in any way possible as they will always be fighting an uphill battle. Due to their low kill rates, Isarelys/Palestinian Auxiliaries are best used to distract and tie down enemies while more damaging troops strike from the flank or rear. Train some of these guys, but only the bare minimum of what you need to survive.
Beylik Janissary Musketeers
The Ottomans have the unusual advantage of being able to obtain their "elite" troops very early. They have the equally distinct disadvantage of these "elite" troops sucking tremendously. Beyliks can't use bayonets, rank fire, or square formation, putting them at a disadvantage in some ways against even Isarelys. However, they do have better shooting capabilities then Isarelys, and have the useful function of boosting nearby troops' morale (something many Ottoman regiments could definitely use). Naturally, they're more expensive to train and maintain than Isarelys. I personally never bother with these guys until my empire's financial situation is stable, but they can work.
Azzars, Haydut Irregulars, Bosnian Panduks, Afghan/Kurdish Hillmen
Without DLC, Azzars are your only early option for skirmishers. If you play my strategy, you'll be losing Bosnia for a bit early on and Azzars will be your only choice. Hayduts and Panduks are better than Azzars, with Panduks having superior melee capabilities and Hayduts having better accuracy. They can only be recruited in Bosnia, however. As you expand east, you'll also get access to hillmen, although there's no compelling reason to use them over the other choices.
Hayduts in particular are quite useful, and are very nearly the best light skirmishers one can obtain in the early game. Having a couple of these to soften up enemy lines before they engage your Isarelys will greatly increase your chances of winning battles with minimal losses.
On a side note, when left alone on the world map for two turns or more, Hayduts and Panduks can deploy devastating Improved Fougasse, which can prove to be the critical difference between victory and defeat against the Ottoman Empire's quantitatively and qualitatively superior foes.
These guys are awesome. Their ability to fire whilst in the saddle is extremely potent, making them better than any other cavalry in the early game by a long shot. They don't have much ammunition, but they usually have enough to last a battle. They are the perfect complement to your Isarelys. On the flip side, they have mediocre attack values and awful defense. Don't get them bogged down in a melee with infantry, or they'll get torn to shreds.
I hate camels. They're worthless, and fall apart like so many bits of confetti. Mamelukes are an interesting choice, but offer nothing much when compared to Deli Horsemen. Wallachian Boyars are interesting, and superior to Mamelukes; they're cheaper in the long run, and represent some of the best melee light cavalry the Ottomans have to offer. As a plus side, they don't cause mass friendly casualties that Deli Horsemen are sometimes capable of.
These melee guys are quite good at defending walls. There are probably better ways you could be spending your money than recruiting these guys, as they are at a severe disadvantage against missile troops on an open field.
Available only in (surprise!) Armenia with the DLC, Armenian Archers are excellent support troops, particularly against cavalry and melee infantry. Place them behind Isarelys and they'll handily fire shots over their heads. This makes the total killing power of your lines actually somewhat passable--especially if the enemy tries to charge into a melee, since Armenian archers become more effective the closer enemies get. They're pretty bad in a melee, though.
An extremely circumstantial piece of artillery. Sometimes it'll kill everything, and sometimes it won't hurt a fly. At close ranges, it'll consistently work as a form of super canister shot, especially considering how organ guns have an extra gun (on large settings). Purchase at your own risk, they're quite expensive.
Artillery in general
Expensive, but potentially necessary. Use geography to make sure that your shots achieve maximum effect. Integrating artillery with canister shot into your Isarely lines will greatly increase aggregate killing power.
All other units are either awful or have a better version listed above; do not recruit them.
Late Unit Overview
This is where the fun starts. Ottoman late game units are equal to, and sometimes better than, their western counterparts. Analysis below:
Nizam-I Cedit Infantry
I am so thankful that these guys exist. They are line infantry equivalents that are BETTER than most line infantry. They are some of the ONLY Ottoman units that can use Fire by Rank and square formation. Spam these guys as soon as you can train them, and delete all your Isarelys if possible.
Beylik Janissary Musketeers
These guys still suck, but they still offer a morale boost. If you find your armies constantly fleeing, then train a couple of these per army.
If possible, replace your Beyliks with Cairo Janissaries. These guys are simply better in every way, and while they do not have the rank fire, bayonets, or squares of the Nizam-I Cedit Infantry, they make up for it with better innate statistics and morale boosting. The only downside of these guys is that they may only be trained in Cairo, Egypt.
Nizam-I Cedit Light Infantry and Riflemen
The light infantry are largely inferior to Hayduts, however Hayduts can only be trained in Bosnia while Nizam-I lights can be trained everywhere. The riflemen are, naturally, unrivaled in range and accuracy. Like other riflemen, though, they're worthless in a melee.
Armenian Archers, Beylik Hand Mortar Janissaries
Armenian Archers are still very much relevant in the Eastern theatre, although they become less important in the West where melee and close range combat is not very common. Together with Hand Mortars, they can make approaching your Nizams an incredibly painful thing for the enemy to do. Hand Mortars are in many ways similar to Armenian Archers, but much better against static enemies and useless against moving ones, making them excellent complements to fill out weaknesses. I would ideally place them behind your Nizam-I lines, but in front of your Armenian Archers. The combination of these three units is a potent mix against melee-heavy armies.
Deli Horsemen are still king. Mounted Nizam-I look good on paper, but can't fire unless they dismount; when mounted they are below average light cavalry that tire out easily, and on foot they're just weaker, smaller Nizam-I Cedits.
Sipahis are heavy lancer cavalry that are initially quite good but tire out easily. Circassian cavalry are extremely good, more powerful version of cuirassiers, but are costly and again tire out easily.
Libyan Koluglu are fascinating, totally unique units. Their blunderbusses make them extremely effective within their tiny effective range. Treat them like grenadiers with horses. Kuloglu are perhaps most deadly when defending a fort. Position them on walls and at breaches for their shots to have maximum effect.
Largely the same role, with a couple of exceptions: Bombardment Mortars are ridiculously deadly, but almost equally inaccurate; 64-lbers are a waste of time that literally can't hit the broad side of a barn, and can't use canister shot. However, they're unparalleled in terms of levelling enemy buildings and walls.
Everything not listed is frankly not useful enough to justify using over what's listed here.
The first turn for every faction is always crucial, as every decision made compounds into the future. I decided from the outset that I was going to switch to a republic due to the horrible ruler and cabinet the Ottomans start off with and from the bonuses in town wealth growth and research rate, both of which the Ottomans desperately need in the early game. The first, easiest choice is to rearrange the ministers so that the best possible stats are available. Destroy the mosque in Anatolia and the coffee house in Egypt, to make room for schools. Offer Venice Bosnia in exchange for Morea (this will prolong peace with them and eliminate one front on where you have to station your troops). Disband all Semenys, Bashi-Bazouks and Musellims (which are nearly useless in combat and eat up your very meagre income). Destroy all fishing ports to make room for trading ports; for what trading ports there are, start building dhows which are to make a beeline for the trading spots. Offer France Mesopotamia. 20 turns of military access, and all your techs in exchange for physiocracy, 8,000 gold and trading rights. Not only does this give a huge boost in funds, it also mostly cuts hostile Persia off from you. France is also going to lose the region shortly due to unrest, where it will declare independence as the Mamelukes, which can be useful in any way between being a buffer state and becoming a stalwart ally. Raise taxes to maximum; exempt all regions except the home region from tax (they're going to be making next to no money anyway, so it's a small price to pay for much faster town wealth and new town growth). Spend any remaining gold on upgrading cultural structures in major cities to make way for observatories. Move the European army towards Crimea, which will need defending from the Russians soon. Sell your technologies to anyone willing to buy; make as many trading agreements as you can (normally this might not be a good idea, but considering the scope of foreign goods you are aiming at getting, the more markets you can sell to the better).
I don't know if this was supposed to happen, but I tried selling the Barbary States my techs and they counteroffered to give me a territory (Algeria) in exchange for my techs and unlimited military access. Needless to say I accepted.
Build schools and trading ports. If you run low on funds, try selling Serbia to Prussia for a good price. This creates a barrier against Austria.
Nothing past turn 1 is completely set in stone. Worst case scenario, Venice, Persia, Austria, Poland, Georgia, Dagestan, and Morroco all declare war on you on top of already hostile Russia. Persia shouldn't be too much of a problem thanks to the Mamelukes that are about to emerge. Austria and Poland can potentially be a huge pain if they aren't busy killing the crap out of each other (they usually are); proper management of your existing army in the Balkans should, believe it or not, hold off their assaults. Georgia and Dagestan aren't that big of a threat in the early game; as a bonus, if Georgia is wiped out early, they have a school in their territory. If the Morrocans end up at war with you, it's probably because they declared war on the Barbary States. Should this happen, it's better to ditch the Barbary States. Keep those trade lanes open at all costs! They are your lifeblood in the beginning of the game. Venice is a huge pain to deal with because they are relatively hard to wipe out due to being right next to Austria, itself a considerably larger threat.
I was quite lucky and only Georgia and Dagestan declared war on me. I wiped them out with a small army of Isarelys and Armenian Archers.
Turn 3 and onwards
The revolution should either be here or be here soon. Make sure you have no troops in your home region when it happens, otherwise there's a good chance they're going to be your enemies. Enjoy your new government with a better than incompetent ruler and better research rates!
Build schools in as many places as you can get away with. Focus on army techs, agricultural techs, and enlightenment before working on the others. The more agriculture and enlightenment you have, the faster your regions develop and become actually meaningfully taxable. Schools cost an arm and a leg but pay off immensely in the long run. On a similar vein, build observatories everywhere. Coupled with the revolution to a republic, and you will have an insanely fast research rates and lots of simultaneous research going on. Sell your techs to enemies of your enemies and use that money to build more observatories and upgrade your schools.
Of even higher priority than schools are trade routes. Capture as many routes as you can (one ship on each node, fill them up later). Build or capture as many 5th rates as you can for defense against those damn pirates. Declare war on the Knights of St. John and capture their fleet if you have to. Managed properly, trade can bring in tens of thousands of gold per turn.
Keep the Crimean Khanate alive. Not only does letting it fall allow Russia to sell its goods to everyone and make it an even bigger threat than it is already, it allows Russia to build ships, causing all sorts of pain for you in the Mediterranean. Selling technologies to the Crimean Khanate and the Barbary States not only pours money into your often-depleted coffers, but also strengthens them in the long run.
Build minimal training facilities, and train the minimal amount of soldiers needed to survive. Save that money for your economy. Focus on farm buildings and school buildings.
Going on this vein for the next 10 turns or so, and the empire should emerge from its cocoon of decaying decline a refreshed and economically powerful force. From then on the campaign is a cakewalk, in part because of territorial expansion diplomatic penalties not occurring for some reason for the Ottoman Empire. You could take out Russia in a long war across its empty lands; you can (you have to, actually) steamroll Persia, perhaps paving the way to India; you could kill Spain and Morocco and renew the Islamic crusade into Christian Western Europe; you could kill off your hated Polish and Austrian rivals to the North. You could wipe out the Pirates if you have the spare resources and establish a colony in the Americas. Nothing really rivals you at this point.