As the game everything is good really. It has nice art, and (mostly) obvious mechanic. Yea, the part that you can move the camera and you need to defeat last castle is wrong in many ways. Just try next time to make this stuff understandable to player even without tutorial.
However, from the design perspective, the game isn't very original. I've seen that type of games even before this gamejam. There is also a AAA project, that replicates same gameplay, but in urban setting.
I know the "defeat the castle" objective isn't that abvious because it has no health bar (which I'm working on and it'll be added to the post-jam version), but I don't know how I could make the camera control move obvious then literally writing it in the controls.
I know it's not original, but my friend insisted on making a reverse tower defense game and it's a gamejam entry afterall we didn't want to waste too much time on coming up with an idea :P
You don't need to be absolutly original. You are making a good work, so keep it up! :)
Advices for postjam:
About castle - either add health bar or make castle more destructable (less health or visual feedback of it's destruction)
About camera. I recommend you to either cut this feature entirely (making smaller levels that fit on one screen) or redo controls - add arrows at the sides of the screen, and allow control camera position with cursor (just like any RTS do it)
