Here is the full list of all 9 Isle of Arrows achievements.
Reach Wave 40 without losing any lives
Build a 3x3 Garden
Playing as the Founders Guild, cover all grounds on the isle
Build a straight path of 15 tiles
Hold 99 coins
Build a lake consisting of 20 water tiles
Build an isle with a width of 40
Fully surround an Arrow Tower by water tiles
Have a spike trap place spikes on all of its 4 sides