The opening is the best sales pitch the game has. You play an elite marksman, crouched behind a scope, picking off targets while the camera drinks in slow-motion kill shots. The store pages promise over a hundred missions across varied battlefields, an arsenal of collectible and modifiable weapons, and sound effects and slow-motion shots that make each battle feel intense. The controls are simple. You line up the crosshair, account for distance and the odd environmental wrinkle, and squeeze the trigger. One reviewer calls the shooting the best thing in the package, and also the smallest part of it. That tension runs through the whole experience, and it's worth knowing about before you commit. 
 

The strategy layer takes over soon after. What begins as a shooting gallery becomes mobile strategy business, where you upgrade buildings and hire new troops to bolster your army. If you've played Rise of Kingdoms, Last War or their many cousins, you'll recognise the rhythm. You tap a building, wait for a timer, spend resources, and tap the next one. Your headquarters gates everything else, so its level is the number that matters most. The game dresses this in a modern-warfare setting with tanks, aircraft, infantry and a cast of collectible officers with their own story beats. That writing, and the mission variety, is what one review rated above genre average. It's a good-looking, well-produced game, and it earns some goodwill on craft alone. 
 

Now to the economy, which is where every free-to-play strategy game lives or dies. You asked about Gold and Diamonds, so here's the first correction: Warline doesn't appear to have a currency called Diamonds. The premium currency is Gold, and the guides I found don't mention a separate gem. If you've seen "diamonds" in a mod-apk video title, that's a sign to close the tab. Those videos are the usual bait for malware and account bans, and I'd steer clear. What the game does have is a trio of headline resources. The main economy revolves around Steel, Energy and Gold: Steel is mostly for developing your base, Energy works alongside Steel for important upgrades and other activities, and Gold is the premium currency that deserves much more careful handling. Layered on top are specialist currencies for recruitment, officers, equipment and alliances, and those shouldn't be treated like your everyday stockpile. 
 

Steel and Energy are the bread and butter. They flow in from your buildings and from rewards, and they drain into construction, research and troop training. The most useful discipline I can pass on comes straight from the resource guides: don't spend a resource just because you have enough of it, spend it when it contributes to your next important upgrade. That sounds obvious until you've watched a friend blow a fortune on a low-priority building while the headquarters upgrade sits waiting for the one resource they burned. Plan two or three upgrades ahead, and let the plan decide what you spend. 
BlueStacks

Gold is the interesting one. It's scarce, it's what the store sells, and it's what the game most wants you to burn. The same guide warns that one of the biggest beginner mistakes is spending it carelessly. The temptation is to use Gold to finish timers the moment you hit a wall. Resist that. In this genre, speeding up a five-minute upgrade is the single least efficient use of a premium currency, because the timer was going to expire anyway. Better uses are the things you can't get any other way: long build queues where every hour saved compounds, limited-time events that pay back more than they cost, and recruitment or progression items that gate your power. Treat Gold like a savings account that you occasionally spend on something that changes your trajectory, rather than pocket change. 
BlueStacks

Free Gold exists, and the game hands out more of it than you'd expect if you go looking. Alliances are the best source. Alliance gifts hand out crucial resources like Silver, Gold and VIP points just for signing in each day, and the quality improves as the alliance levels up. Redeem codes are another. At the time of writing, the code WARLINEOBT was listed as granting 1,000 Gold, 50,000 Steel, 50,000 Energy and a 15-minute building rush. Codes expire, so check a current list rather than trusting my memory. The same source is honest about what they do: you'll still hit the same wall, they just delay the collision. That's a fair summary of the whole free-resource economy. It's a cushion, not a way around the design. 
Warline Sniper Strike Alliance Guide - Unlock Requirements, Rewards, and Game Modes |  

That design is a business, and it's where the packs come in. The store sells bundles rather than bare currency: recruitment, troop tier upgrades, speedups, research boosts and Gold all flow through the store as packs and bundles. A typical pack mixes Gold with recruitment orders, speedup time and resource crates, and it's priced so that the bundle looks far better than buying any single item. Some are one-time starter offers, some rotate with events, and some are timed to a moment when you're likely to be stuck. From my reading, the pattern is the classic one: a low-priced first-purchase pack that's genuinely good value, followed by steeper offers that scale with how deep you are. If you're going to spend anything at all, the small early packs are where the value is, and the $99-style mega bundles are where the game's whales live. That's my read of the genre rather than a figure from a source, so check the in-game store for actual prices in your region. 
 

I'd be doing you a disservice if I glossed over how divisive the spending is. The public numbers are glowing on paper. The game holds a 4.9 rating across more than 53,000 Play Store reviews. But dig into the written reviews and the mood changes. One reviewer complained that the game is a money grab. Another described spending more than six hundred dollars just to get a main building to level eight and then being forced to relocate after other players ganged up on them, with the relocation costing more Gold than a hundred-dollar pack provides. That's one person's account and I can't verify it, but it lines up with the shape of the complaints elsewhere. Competitive strategy games reward spenders, and Warline is no exception. If you're a free player, you can have a great time, but you should expect to be outpaced by anyone who opens their wallet.