I laughed a lot with this game, especially towards the end (the button that activates the falling celing) - I initially thought it was a trap door, not a button, and jumped... only to be stabbed in the head, awesome.
I think the following improvements are needed to make this game better:
1. Improved tower AI, so they're not always attacking the "furthest" monster, and will distribute damage to where it's needed, I hate seeing a sniper tower fire off at a monster with 1% health. - possibly also add the ability to select "weakest / strongest" targetting options.
2. Increase tower sell value - the money return of selling a tower is way too low at the moment.
3. have the +range and +damage towers stack their effects, so that building more than one of each is worth doing.
I see those as the only neccecary changes, but would also like to see a fire rate boosting tower (possibly auto-unlocks when all other upgrades have been purchased)?
it's a good concept, but the standard rifleman unit is far too strong, having 3 or 4 of those firing at an enemy castle (even on hard mode) will cause almost guaranteed success.
clear out enemy troops, send in 3-4 gunmen, and things will die before they can move, castle will crumble as AI continues to build buildings.
other than that, I really liked it!
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Its beatiful, I know.