To start, I want to say I'm enjoying the game so far. I've put in about 25 hours in the first two islands, mostly trying things out and devising strategies (unrelated, but I'd love to see a sandbox mode).

This is something I noticed while playing Island 2.

Setup:

1. Build a city on one side of the island (Town #1)
   
2. Build a maze around the gate
   
3. Block off part of the map by building your defense structures all the way to the map bounds, such that the AI path can only go through the maze and to Town #1
   
4. Build more civilian buildings in the area you blocked off (Town #2)
   
5. Try to build (not replace/upgrade) more defense buildings. No matter where you put it, it will say that you are blocking the enemy path

My intent with the split town was to have housing & resource generation closer to the further reaches of my maze. As far as blocking the map off, my intent was to increase the path length and tower density to get as much DPS as possible. Both, IMO, are reasonable strategies.

However, if enemies slip through and make it to Town #1, they will sometimes glitch across the map to Town #2 to attack it. When I was playing, the Boss glitched through the volcano, into the ocean/bay, and then to the other side to destroy a storage barn.

The workaround is to destroy all your blocked-off civilian buildings. At which point, you can continue building new defense structures. It works, but might not be the best UX. You don't find out you can't build civilian buildings like that until it's too late. It's like there's an invariant in the pathing logic that says all civilian structures should be reachable by pathing, yet the logic for placing civilian structures is missing this check to maintain the invariant.

IMO, either a user should not be able to place these blocked off buildings in the first place, or the enemies should have some built-in mechanism to reach those buildings so they don't go glitching (ie. they can destroy walls when blocked off buildings are detected, or maps have a 1-unit non-buildable border around them to allow AI to always reach any part of the map)