I've seen a few games that have a similar idea in concept. This one does a great job in executing it. 

We kinda made the same game: Reverse Tower Defense with Goblin attackers. There were multiple others, so I made a collection.

I like how the different speeds of the units matter. If it was a little bit easier to swap units and spawn them in a selected lane, I think there would be a lot of potential for timing strategies, so you could for example send out an Ogre that you protect with precisely timed goblins etc.

Cool idea and good job!

Cool game! I can see the roles reversal in the way you implemented the tower defense form the opposite perspective, good job!

Nice Game! It seems a bit unbalanced at this stage, after a certain point I found destroying the main tower borderline impossible because of how overpowered the cannon towers are. However, the variety of enemies you can spawn and the different towers you can encounter still make up a very interesting game that I would love to see expanded further. Good Job!

Very fun mechanics, some descriptions might be needed but this was fun nonetheless.

Nice game, but when the tower combine it will be the best if you add the icon together, i got confused at first that normal tower can shoot cannonball before knowing it can be placed on top of another tower.

and the difficulty spike is hard, I can't finish the level 5.

but yeah great game, manage to keep me playing until level 5!

Nice Game!