I've played through a few levels, not really knowing what I was doing, but eventually winning and not being really sure why. The last level that I finished was the only one that really gave me trouble. I don't remember the stage number, but was the first time I saw an enemy castle with red balloons that went out and picked up red mana, and had an enemy wizard defending it.

The spells I had available were conversion, fireball, castle, accelerate forwards, and accelerate backwards. I'd scour the land for mana, blasting enemies with my fireball. They'd drop gold mana, which I and the enemy sorcerer would both try to convert for our balloons to scoop up. When it looked like my balloons weren't leaving the castle any more, I'd return to cast castle on it and watch it grow. Occasionally I'd take potshots at the enemy sorcerer with my fireball spell, especially if he started attacking my castle, but for the most part he was moving too fast for me to do real damage to.

At one point, a huge swarm of archers just spawned around my castle and trashed it, spewing mana everywhere. I killed them all, rebuilt, and re-upgraded as much as I could.

Eventually, I couldn't find a single black dot on the map. I'm guessing that all of the mana was in the two castles. So I started blasting the enemy castle with fireball. It took a long time, and involved lots of rapid clicking, but eventually it shed a layer and bled some red mana. I'd convert as much as I could and defend it from the wizard until my balloons could steal it. Before long, he'd come by and re-upgrade the castle, which tended to absorb any red mana that I hadn't converted yet.

Repeat the process several times, and eventually the castle stopped bleeding red mana altogether. And the wizard would repair the castle before I could do any further damage.

Here's what turned the tide. By what felt like sheer luck, I managed to kill the wizard. I'm not sure how I did it. With him dead, I had enough time to fireball away two layers of the enemy castle. The second layer bled a lot of red mana. I converted a big chunk of it, my balloons got some of it before he could take it back, and that triggered the level end.

I'm sure I was doing something wrong. This was way too tedious to be the way you're intended to win. What are you supposed to be doing differently? What am I missing?

That's pretty much it. You can make it a bit less of a slog by keeping pressure on the enemy wizard(s) from early on in the level, rather than letting them amass so much mana, then having to wage a protracted war of attrition against them and their castle(s). Also, look for ways to pit your enemies against one another (enemy wizard vs. enemy wizard, or enemy wizard vs. monsters). Some of the spells you'll get later are a little more impressive than simple fireballs, too. ;)

Thanks for the tips. I'm a good bit farther ahead now, and I just want to add some tips that I wish I knew in retrospect.

One other thing that really wasn't (and still isn't) helping me is that I'm colorblind, and red dots don't stand out against the terrain on the minimap as much as they should.

Before finishing a level, quicksave with Alt+S. Then if your "spells found" count isn't 100%, go to the next level and press Alt+L to quickload, and search for the missing spells. There are some spells you REALLY want to make sure you get in the earliest level possible, especially rapid fire, lighting, and meteor. The particular level I was having trouble was a miserable time because I was stuck with the basic fireball. Rapid fire would have let me kill two levels of castle on one mana bar, without having to kill the enemy wizard.

You can upgrade your castle as long as you have enough mana, even if the mana isn't in the castle yet. So can your opponent. This means that when assaulting a castle, you should focus on stealing mana rather than maximizing damage to the castle. If you damage the castle and it starts spilling any significant amount of mana, stop attacking (unless you think you can finish off the castle) and convert it.

Also because you can upgrade your castle as long as you have enough mana, you don't need to wait for your balloons to carry it back. If your castle is close to maximum capacity and you've converted a big load, protect your balloons until they can collect it but return and upgrade the castle BEFORE the balloons return. Otherwise your excess mana is going to spill and then you'll waste time waiting for your balloons to scoop it back up.

Mana recharges faster when you are in your castle.

In the particular level I struggled with, at some point skeleton archers spawn en masse and trash your castle. When this happened to me, the enemy stole most of my mana, to the point where there just wasn't enough mana left for the level to be winnable without tedious tactics. So I restarted and triggered the skeleton archers right away, and let them destroy my level 1 castle. By the time I killed enough of them that the situation was under control, the enemy still grabbed most of their mana, but the situation was less grim overall.

I'm certain that if your castle is full, then your balloons will not leave to collect more mana. I don't think balloons will pick up more mana than your castle can hold if they are already out and about when the castle fills up, but I'm not as sure about this point.

Your castle will auto-grab any mana you own that is within or very close to the castle (roughly within the square of land that auto-levels to match the castle's elevation), up to your castle's capacity. Enemy castles will do the same thing for mana of the castle owner's color.

You can build your castle on water; if you build it far enough out to sea, then land only creatures (like skeleton archers) won't be able to damage it, but sea creatures (krakens) may attack it. The opposite is true, in that sea creatures can't attack a land bound castle. Flying creatures don't care about your petty attempts at isolation. Undeformable terrain (like some steep cliffs), buildings, and enemy castles can block you from expanding your castle; if there isn't enough room for it to grow to the next size up, then casting the castle spell on it won't have any effect.

When you upgrade your castle, it will hurt creatures (not wizards) that happen to be in the newly expanded castle's AOE (about the same AOE where the castle auto-grabs mana of your color). This will outright kill lesser creatures like vultures, bees, wyrms, and dragons, but stronger creatures (wyverns, griffins) may survive. I haven't successfully killed a wyvern with this method, though I think I've finished off a griffin with it.

When you are "inside" (hovering over) your castle, your wizard will not take any damage (it is all applied to the castle). You also regen health and mana faster while over your castle.

It is possible to destroy your own castle; I think the hotkey is Ctrl+L. Using the hotkey will destroy it one level at a time (zeroes the health of the current level), and removes it when used on a level 1 castle. This is mostly useful if you realize you can't expand it anymore (see above about blocked expansion).

Correct, but if the castle is 99% full, then it will send out a balloon, and the balloon will happily pick up a full load. When it does this, if you don't upgrade the castle before the balloon returns, all the extra mana will spill.

I just wanted to add in regards to mana held in castel vs. mana possessed on the ground:
What I figured out is when you have the later fighting castles (fire/lightning towers) if your castle is full of mana and the balloons aren't collecting any more you can usually only build a basic castle upgrade. To upgrade fire and lightning you usually have to possess a bunch of mana on the ground IN ADDITION to the castle being full to pay for the upgrade.

Further, your mana 'stamina' bar grows if you are possessing mana on the ground. You'll find that in places where you can't build a castle, if you capture a lot of mana your spell selection and mana stamina bar will grow anyway.