Great game!
I can't understand all the complaints about difficulty - it's a bit easy mostly. It's true lvl5 is a bit tougher, but it's the only level I take any serious damage. Hints: stick to upgrading fireball speed then power, and get the towers one by one as you can afford it. Then upgrade arrows, moat, upgrade arrows/fireball to max, then barrier, castle. You do NOT need the moat to survive lvl 5.
Aim carefully with the fireballs, leading the target is essential, and at early stages "fire and forget" by only let off enough shots to kill the target then aim elsewhere. If a powerful enemy e.g. giant is far away then let off a few shots, check the rest of the area, then send a few more shots at the target. This will prevent other minor enemies sneaking up on you and you becoming swamped. Once you have towers keep your finger firmly on "S" and saturate the whole area - with 3-4 towers most enemies will have taken a pounding before they get too close, and finish them with a few well placed fireballs.
Please make Castle Siege 2 with more options and more challenge etc. Where are the death cries? There may be a lot of castle defence games out there, but this is one of the best.