One of the things you can spend gold and BP on in Tower Wars is to upgrade your mines which lets you make more gold per turn. On the surface this seems like the most important thing to me. Spend your money to make more money! Then when you have an enormous income use it to crush your opponent underneath your massive piles of gold! Unfortunately it turns out that if you take the time to just level your mines your opponent is free to spend his money on units instead. This will force you into spending money on defending yourself on top of upgrading your mines which means you won't have any money left over to attack him. This means your opponent can skip spending money on towers AND it means he has a source of BP income and you don't. So while you'll end up with more gold than him eventually, you're also going to end up dead. It also turns out that if you spend all your BP on mine upgrades instead of on unit upgrades your opponent can just front load their maze with high level towers and it becomes impossible for you to generate enough BP. You need to spend BP to make better units to survive that onslaught, but you can't earn BP without units that can survive in their maze for at least a little bit.
All this means that just pounding out mine upgrades is a bad idea. You need to pace yourself. But when does it makes sense to upgrade your mines? Is it possible that skimping on upgrading your mines can be the way to go? My experience tells me that falling behind in mine upgrades and then trying to catch up is bad because you're taking time off from attacking and getting less out of it than your opponent did. So he just gets an edge. Maybe trying to all-in him is the right play? But then he should just get a small enough mine upgrade edge to be relevant and then switch off to defending himself and attacking you to force your money into towers too. It's all so very tricky...
The first step is going to be looking at what actually happens for you when you upgrade your mines. Every map seems to have exactly three mines on it for each team. You start with level 1 mines that each have 1 worker and every mine can hold up to 6 workers. If a mine has all 6 workers you can pay to upgrade the mine a level and then you're allowed to buy 6 more workers. There are 3 levels total.
The level 1 workers cost 25 gold and 15 BP and generate .65 gold per second. This means a given level 1 miner will pay himself off in 39 seconds. My experience tells me the gold cost, while real, is not the relevant part here. You need to earn BP to buy the workers and the BP is harder to come by. Sending an initial wave of 7 guy seems to generate around 60 BP, so you can earn enough BP to buy 4 workers by spending 175 gold. In a sense I guess it actually costs 275 gold for 4 workers? At least at the start? Regardless, that would still pay off in 106 seconds and that feels like it has to be worthwhile.
Level 2 workers cost 100 gold and 45 BP and generate .8 gold per second. But before you can buy them you have to pay 375 gold up front. So the first miner costs 475 gold and even ignoring the BP it would take almost 10 minutes to pay that guy off. Being down gold for 10 minutes is absolutely terrible. Who cares that you're ahead after that point, you've probably lost! But all that means is upgrading the mine to build a single worker is a bad plan. Realistically you should be planning to do a full mine upgrade all at once, and that costs a total of 975 gold and 270 BP to generate 4.8 gold per second. That pays off in 204 seconds (ignoring BP) which is an amount of time that feels reasonable. I can see playing down for 4 minutes in order to get ahead for rest of the game if the game is going to take 20 minutes, for example. Spending that money is going to set you back on BP generation and give your opponent a reprieve from attacks, but I can imagine how that's a reasonable thing.
Level 3 workers cost 300 gold and 90 BP and generate .95 gold per second. The cost to upgrade the mine is 750 gold. Once again the first miner is a terrible investment since he'll only pay off the gold cost in a little over 18 minutes. A full mine on the other hand will pay off in 448 seconds. That's actually a pretty long time, especially since the 540 BP cost could unlock a new support unit.
One thing worth thinking about is how much gold you're going to be spending on a typical attack wave. If you can get your gold income above the amount you're spending on attacks then you have left over money to invest in upgrades (either more defense, or mine upgrades, or on better units) while keeping your pressure on. At the start of the game you can send out at most 18 guys that cost 25 gold each. You can send a wave every 45 seconds. So your start of game gold expenditure on attacks is capped at 10 gold per second. A full set of level 1 mines will make 11.7 gold per second, and you will also earn money from units when you kill them. If he's sending out the first unit you get 10 gold for killing them, so you get an extra 4 gold per second from them. So you've got a pretty large surplus running to use saving up for an upgrade of some kind. It would take about 3 minutes before you could afford a full mine upgrade, but that's assuming no money is spent on towers and that's probably not true.
Regardless, the first upgrade you're going to get is almost certainly a better unit. The next unit in the chain costs 250 gold and 250 BP to unlock, and increase the cost to send a wave to 513. That's still less than the cost of a full set of level 1 mines (barely) so you can use the bounty on killing the enemy units to buy more towers and save up the initial 250 unlock cost from the earlier profit. That seems pretty good, but then you're stuck without really making money to go any further unless the bounty from unit kills is a net profit (I feel like it probably isn't) or you take time off to upgrade your mines. You lose pretty much a full wave and a couple early unit upgrades to do it, but it will pay off in under 4 minutes. I feel like at some point once your opponent has stabilized a little you're going to need to do this to advance further?
The next set of units are a fair bit more expensive than the second tier. They're also support units, so sending a full wave of them feels wrong, though looking at the stats they actually have comparable health per unit slot as the second tier so maybe I should try sending waves of just tier 3 units for a while and see how that works out. Anyway, a full wave of support units will cost you 828 gold and you have to spend 500 gold and 500 BP to unlock them. Comparable health but way more expensive than tier 2? I guess that's a reason to stick with some tier 2 units. And at a cost of 18.4 gold per second to send a full wave it's actually not viable with only level 1 mines.
The next tier up is even more extreme, with a cost of 1458 for a full wave. The level 1 mines can barely support a 513 cost! Almost tripling the price is right out, especially with the 750 gold and BP cost to unlock him. He does have more than double the health of the tier 2 wave, so you are looking at getting your money's worth out of him. Especially when your opponent's maze suddenly needs to deal twice as much damage as before! Getting these guys out and being able to spam them is a big deal and should cost your opponent a lot of gold on his towers, which should buy you some spare time.
The problem? A full set of level 2 mines will only generate 26.1 gold per second and sending full waves of tier 4 units costs 32.4 gold per second. You probably don't need to spam them on cooldown since they're pretty good, but if they don't win you the game then and there you're kind of stuck. You may have to take the time off to upgrade to level 3 mines at that point?
The way long games seem to end is someone unlocks the ultimate unit and sends a few of them out. I've lost after sending only 3 (you can max out at 5) because of how much gold I sunk into not winning the game. But I bet if I'd saved up a little more money and sent the full wave I would have won. What does it take to do that?
You need to unlock all the tiers of units, though by the time you're thinking about this you should have the support tier at least. I often don't get the 4th tier, but maybe I should be. Anyway, the cost for tiers 4, 5, and 6 combined is 3000 gold and 3000 BP. You may need to fully upgrade him, which is an extra 3232 BP on top. And then to send a full wave (which you have to spend other upgrades to unlock) is 8060 gold. So you need to have a huge stash of BP and then save up 11060 gold. At 26.1 gold per second you're looking at 423 seconds to save up that much cash. Alternatively if you have full level 3 mines you're making 43.2 gold per second and it only takes 256 seconds to save up the money. You can save 167 seconds by getting the level 3 mines. The problem is it's going to take 293 seconds to save up the money needed to buy the level 3 mines! (It will actually be a less since you don't need to save up the full cost up front here, and the extra miners you get along the way will do something. Just not enough!)
So it feels like if you're at a bit of an impasse and want to do something to break things open... You may be better off saving up for a full wave of ultimate units instead of getting more mines. I guess it depends on if you can earn the 6232 BP? The mines do only cost 1620 BP...
I think I'm going to try playing a few games with a focus on getting to full level 1 mines, then just work on spamming tier 2 units. At that point mines are probably better than tier 3 units, unless I think I can win with a few support units. And I'm going to try to avoid upgrading to level 3 mines. I'll try out the tier 4 units and then just save for what I hope will be one 'I win' push.