Graphic stuff is a bit messy (I loved the 3d turrets too, but this is a limitation I might have to accept and might not affect gameplay) so I'll comment on the gameplay parts
ddabrahim wrote:Regarding the actual gameplay one thing that I would like to suggest is to pick the enemies that are closest to the end of the road or whatever need to be protected or closest to the turret and keep the turrets focusing on that while they can...
You're right, they are simply targeting the closest enemy right now. So if an enemy passes, another enemy might become closest and then the turret will "forget" about the passed enemy, and the enemy will pass unless there's another turret for which it is the closest. It's not ideal for sure, but it's the simplest to make. If it makes the gameplay too difficult or silly, and thanks for the advice, I might have to change that so that it keeps following a single unit until the unit is destroyed or out of range. But this could cause other problems, e.g. what if a turret that cannot really defeat a target (e.g. for now the first turret can't hurt the large enemies in level 2, and in the future maybe some other types of vehicles will need specific turrets) then this turret shouldn't keep shooting at this target, because then it will be useless. Of course, more logic can be added so that turrets target only the proper enemy types, but it is one more layer of complexity. As for selecting strategies (e.g. closest, strongest, etc) that is fun for players and I have noticed the selection option in some games, but then some of the greatest tower defense games (gameplay quality wise) don't have that selection, so it's worth considering whether the additional work would improve gameplay significantly or not (to see if it's really worth it).
ddabrahim wrote:Also you might need to consider to let the player start the wave whenever it ready with the turrets
Definitely, this and possibly those speed up/down options are not difficult to implement. I just didn't implement them yet. I'm still exploring how feasible these projects are in Gdevelop, and it helps a lot to keep things as simple as possible. You don't have the object oriented language organizational advantages (objects, or even functions in HTML 5), but on the other hand simpler tasks (without too much complexity) are likely easier in Gdevelop, although it's hard to compare as I didn't make many projects of this type at all.
P.S. you don't necessarily need an engine that supports 3D models, you could have one large image spritesheet for each pseudo-3D object, and one large image is much faster to load than 360 separate images.