The "Basics" of Prismata
Here are what I consider to be the most basic principles of Prismata strategy. Once you read and absorb this, I'd say that you officially know "how to play Prismata". Do note that I'm assuming you have some minimal level of experience with actual play - you should at least play a game or two of base set only against Adept Bot before you come here. Finally, to see the panels for the units I mention if you don't have them unlocked, go to prismata.net/units and use your browser's text search (should be Ctrl+F and then type the unit's name).
Always absorb ¶
This is the most important concept of them all. Most blockers in Prismata, such as Wall, heal back to full health immediately after the defense phase if they take partial damage. This ability is called absorb and is incredibly powerful. To see why, consider the following: if you defend for 5 and your opponent attacks for 5, you lose all of your defenders and nothing else. If you buy 1 more point of defense and defend for 6 instead, you can lose all of your defenders except the Wall, put the last 2 damage there, and at the start of your next turn it has 3 health again. Thus by buying just 1 more HP of defense you gained yourself 3, because you were able to waste ("absorb") 2 enemy damage on your Wall. For this reason, you should be willing to go to almost any extent to avoid being breached, even for 0, because absorbing enemy damage is bonkers efficient. If you have to block with 3 of your Drones to save your Wall, then that is what you must do. Always absorb.
Always get the biggest absorber ¶
An extension of the above-principle. Prismata is a very well-balanced game, so most of the time, you don't just have to ask which unit is "better", but which unit is better suited to the situation. But one thing that's deliberately not factored into defenders' costs is the power of absorb. Doomed Wall is of a similar cost-to-HP ratio to Wall (about 3% better), but also allows you to absorb 1 more damage for free each turn. Therefore in a Doomed Wall game you should make sure you always have a Doomed Wall available when you're taking substantial damage so you can absorb 3 instead of 2. It's like your opponent has 1 less Tarsier the whole game.
Don't waste attack ¶
A common new player instinct is to blindly attack with everything that can attack, but this can be really harmful if you have units that pay a cost to attack (like Rhino which loses stamina each time or Militia which attacks instead of producing gold). For example, if your opponent has a Wall and you have no attack except two Militias, it would be a very foolish move to attack with the Militias because it does nothing (the Wall will just absorb it) and you could have got 2 gold instead. Always check if your attack will actually kill anything before you attack. In fact even if you have 3 Militias you shouldn't attack into a Wall (because 3 gold is worth more than the one Engineer you'd kill).
This is especially relevant against blockers that only have 1 lifespan left, since they'll die at the start of the next turn whether they take damage or not. For example, if your opponent has a dying Chieftain on defense as well as an Energy Matrix, then the first 11 damage goes to waste, so if you have a pile of Scorchillas, you'd better wait until next turn to fire them (unless you have close to 11 attack even without them).
Sometimes it's possible to effectively obtain free damage by attacking for a number that prevents the opponent from getting their full absorb. For example, against an opponent with nothing but 3 Walls, attacking for 3 or 6 causes them to lose the same amount of defense as if you attacked for 5 or 8 (if you don't see why, play it out in your mind or in game). In this case, you're exploiting for 2 by denying 2 absorb to your opponent. Attacking for 4 or 7 is an exploit for 1. If you have offensive finesse (the ability to control exactly how much damage you deal), keep an eye out for opportunities to exploit your opponent's defense.
Defensive granularity ¶
But what if instead of three Walls you had two Walls and three Engineers for the same total health? Against that defense, the exploit doesn't work anymore - if they attack for 3 you just lose one Engineer instead of an entire Wall. Having a couple of low-health defenders mixed in, called defensive granularity, prevents your opponent from exploiting your defense. The best way to achieve this is by making sure you always have a couple of Engineers or other 1-HP defenders available, and replacing them ahead of time if you can tell you're going to lose them. Defensive granularity is important because it stops your opponent from effectively dealing free damage through exploits.
If you find yourself without any Engineers on the board and you realize you're going to be exploited, what you can do is hold back a Drone to block. If this allows you to absorb one more damage in addition to blocking one itself, it's a strict improvement over spending green on a Forcefield, and a massive improvement over spending 5 entire gold and a red on a Rhino.
Advanced exploiting ¶
Even if you can't exploit the opponent's defense yet, offensive finesse can still be important. For example, if you're about to deal 5 damage into a defense of 2 Walls and 2 Engineers, consider clicking a Militia so that you kill both Engineers instead of one Wall. That way you'll be able to exploit them next turn. I also sometimes call this "pseudo-exploiting".
Deny absorb ¶
There are other ways to stop your opponent from getting absorb besides exploiting. One is to use long-buildtime units like Tarsier as your first attackers - the longer you don't actually deal any damage for, the longer it'll be before the opponent can start absorbing.
That also applies if, say, you've bought one attacker and the enemy responded with a Wall. A second attacker would do nothing until the third arrives, so don't buy just one unit that'll attack next turn if you can help it.
Another way is with units that cost attack or have abilities that cost attack. Take Lancetooth for example. If you have 2 Tarsiers and the opponent buys a Wall to absorb the damage, Lancetooth becomes an insanely good purchase because they already have the Wall ready and waiting to absorb, so the two damage it costs is worth literally 0. Lancetooth is actually balanced around this idea - it's a pretty bad purchase if you're paying 2 real attack for it.
Don't overdefend ¶
You should almost never buy prompt defense before the turn you actually need it. You might be worried that if you don't start building defense ahead of time you'll get taken by surprise and find yourself unable to defend, but this fear is almost never justified. You'd be amazed how much defense you can pull out of nowhere if you really need to in Prismata. So next time you're thinking of buying a Wall that you don't need yet, try buying a Steelsplitter instead. The Steelsplitter will start destroying enemy defenders immediately, forcing them to buy new defenders instead of more attack, leaving you free to do the opposite. You always want to buy units so that they get put to use as soon as they finish construction.
This doesn't mean defenders without prompt are bad. Non-prompt defenders have other advantages over prompt ones to compensate for needing to be bought one turn in advance, like more hp (Infusion Grid vs Wall) or a lower cost (Perforator vs Rhino), and actually tend to be really good; but you still don't want to buy them so that they start blocking before you need defense.
There's often a choice to be made between buying a permanent attacker, like Tarsier, and one that puts immediate pressure on your opponent but then peters out, like Rhino. You might imagine it would be a good idea to mix the two together so you have "some of each", you know? But no.
There's an effect in most Prismata situations I call "the pressure bonus": when we have to buy defense, we exhaust the most efficient source (often Walls) first and only once we run out of blue we turn to less efficient things like Rhinos for defense or holding back Drones. That means the more damage you have, the better additional damage is. That's why you should buy your pressure units on turns when your enemy will already be scrambling for defense. Pressure units are balanced so that they're weak when they force out efficient defense but strong when they force out inefficient defense.
Don't overtech ¶
Another common new player mistake is to buy a lot of tech buildings without enough Drones to make use of them. If you buy two Blastforges, two Animuses, and two Conduits with just 15 Drones, you won't have nearly enough gold to spend all of your tech, and you'll end up wasting a lot of resources.
307th came up with a rule of thumb that I think is pretty accurate: in general, one Conduit requires 3 Drones to spend efficiently, one Blastforge requires 5, and one Animus 8. (The Conduit rule is because while Forcefield only costs 1 gold upfront, it consumes a Drone, so you need more than 1 Drone per Conduit if you want to continue buying Forcefields.)
Of course, these numbers are subject to change depending on the random set. For example in a set with Immolite, an Animus only requires 6 Drones to support.
Use the random set ¶
The random set units in Prismata are deliberately balanced to be slightly stronger than the base set units, to keep each game fresh by switching out the best units. Therefore, base your strategy on the random set units unless you know what you're doing. Don't make an army of Tarsiers in a set with a unit like Hellhound or Cynestra. If you're faced with a random set unit you've never seen before and you don't feel you have a good idea of how strong it is, it's probably better than base set units that fill the same role.
Attackers are better than Drones ¶
If you're coming from a game like Starcraft or Age of Empires, you might have the intuition that you should continue to buy Drones for pretty much the entire game. But in most games of Prismata, after you pick a strategy, you should get the amount of Drones you'll need for it before you start buying attackers, and then stop buying Drones once you start buying attackers.
The reason has to do with the math behind Prismata, but here's a summary: when they're not being absorbed, attackers are straight-up more efficient than Drones; the only reason you buy Drones at first is because early attackers are susceptible to being blunted by a Wall. Since the max amount of damage your enemy can absorb is a constant (2 if they're using Wall), rather than a proportion of your attack, the absorb problem is a bigger deal when you have a smaller economy.
To illustrate, if you go for a turn 1 Blastforge into Steelsplitters, you can make 1 Steelsplitter per turn, meaning you lose two entire turns of production to absorb. If you waited until turn 3 or so and then got 2 Blastforges for 2 Steelsplitters a turn, you only lose one turn of production to absorb. The advantage of waiting to buy attackers is clear. But if you wait too long, your opponent will be the first one to start profitably making attackers. You want to find the sweet spot where the increased efficiency of attackers starts to just barely outweigh the drawback of being absorbed at first.
Even weak attackers are better than Drones if they don't get absorbed. Don't buy two Drones in the late-game if you could buy a Gauss Cannon instead. If you only have 3 gold, consider spending it on an Engineer instead of a Drone. The Engineer is cheaper and will block next turn, allowing you to devote more resources to buying attack instead of defense, not to mention the benefits of granularity.
Big absorb means high-econ ¶
If absorb is the main reason to buy Drones before attackers, it should follow that the size of the biggest absorber in the set largely determines how many Drones you want to get before you start making attackers. In a set where Wall is the biggest absorber, 12-15 Drones is usually enough. In a set with a big absorber like Energy Matrix, you'll normally want around 20 Drones. Try getting a third Engineer on turn 2 in sets with big absorbers so that you can make Drones faster.
Blocking with lifespan units ¶
Attacker-defenders that have lifespan are almost always better off blocking then attacking on their last turn. I've see some new players not really notice that this is an option, but they generally have more HP than attack. For example, Doomed Mech can either give you 2 attack or 5 defense on its last turn. Of course, you should still check before doing this if the Doomed Mech's attack would breach or something.
Overdefend to keep your attackers alive ¶
Overdefending is generally a very bad thing. But if you have attackers that are blocking (Borehole Patroller is the best example), you should buy enough defense so that you don't have to lose them if possible. Borehole Patroller is worth more much as an attacker than as 2 defense. (Another way to look at it is: buying 2 more defense to save it is cheaper than replacing the attack it provides.) So if you have 5 Borehole Patrollers and 10 damage is incoming, you need 21 defense, because you need to be able to not use the Patrollers and still defend. Remember that not every unit that says 'Blocker' on the top is actually meant for defending.
One major exception: Xeno Guardian has so much health for its attack that it's usually cheaper to just replace the attack it provides than to buy 4 more defense to save it.
Freezing lifespan blockers ¶
Chill is extremely effective against blockers with lifespan, because if you freeze a unit that only has 1 lifespan left, the unit dies without ever being able to block, meaning you essentially cheated your opponent out of all that free defense. It's such a hard counter that it's often correct to just attack with lifespan units on their last turn if your opponent could freeze them, or sacrifice them early so that they never make it to 1 lifespan (but that only works against some kinds of chill. Shiver Yeti can just freeze your Doomed Mech continuously).
Verify that damage indicator ¶
You'll often find yourself overdefending if you always defend against the indicated attack from your opponent. Sometimes your opponent is defending with attackers and they're going to have to lose some this turn, which the attack indicator doesn't account for. For example, if your opponent is defending with a Wall and two Rhinos against your 6 damage, they have to lose at least one Rhino no matter what, so you need 1 less defense than it says. There's also the issue that the attack indicator assumes the opponent has enough resources to click all their stuff, so if they have a Zemora Voidbringer but not enough green to fire it, you can defend for 8 less than the indicator says. (It also assumes that all their chill will be put to use without any overflow.)
The attack indicator does discount units that can't attack because they're under construction or out of stamina. I think the rule is that it accounts for internal limitations but not external ones. So if the unit can't fire because the enemy can't pay the cost to fire it, then you have to subtract it manually; if it can't fire because it can't fire, then it's already subtracted from the indicator.
Alright, that's all I consider to be the basics. If you absorbed all of this information (like my sicku pun?), you should be able to beat at least Adept Bot every time now.