So other reviewers pretty much covered it, it is play to pay, for the most part. You don't have to, by any means, but it is hard to get traction (level) if you don't. Starting off isn't so bad, but as you play more you realize leveling your heroes and titans are a real B.
It really depends if you need to move them up quicker or your happy with the grind. Pretty much all mini games are available to you, pay or not. The dungeon, campaigns, tower, arenas, and all special events (which they do a lot of). None of that costs a dime. No change in rewards either (except the season events), but yes, you can throw money at special events to get more, which of course they are hoping for. That's pretty much the business model.
I think the game is pretty playable without spending much. Almost. If you can't stand being a low level, then you will either spend money, or you probably don't want to play. I've done a little of both, stick to the grind, but from time to time, toss some money at the game. So, If you are cool with the time sink grind, then you're fairly okay.
However, I would say regardless how you feel about that, you'll want to throw a little cash at the game any way so you can RAID. This allows you to just click through the battles and get the rewards. Big time saver. Your able to RAID at VIP1, which is for life. So you spend a buck or two (one time) say on some emeralds, and it is yours forever. No future payments. That isn't bad, but here is the hook, VIP1 allows you to RAID one battle at a time. Once you do it a few times, you'll likely wish you can do like 10 battles at a time or more. To do that you will need VIP5. Which means a little more cash. Again, once there, it is yours for life. So not and endless money sink. Me? I consider it worth it. Really removes the grind. Plus, your not spending much to get to VIP5.
A nice optional money sink is the Valkyrie's Favor. It is pretty cheap, but if you do it, it is a weekly renewable cost. Its a nice bonus that allows you to play battle 5x as fast, and gives you a few other helpful rewards on a daily basis. This could also go to your VIP status as well. Basically anytime you send money, it adds to your VIP status. I'm cool throwing some money at the game. I spend more on my daily refreshments, so this seems cheaper by far for entertainment.
With all that said, the game is decent, its not like you will play it all day, just a little bit each day, and move on to something else. This review was for the web based version.