Missile turrets shoot them down. Doesnt the radar detect them?
Yes and and yes, but why use radar in your base if there is no threat to your base

and why built turrets if there is no threat to your base?
Basically you should not have any turrets, if the Toss player does not have a starport or a Dark Shrine.
Also a good player will find a darkspot in your turret defense and put the observer there, and if you have surrounded your base with a gazillion turrets then they will just pop a few around the outside of your base, to see when your moving out, and fly one (or a hallucination) into your base from time to time to see what your building.
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BTW as I have noticed a few people I play with from here build loads of pylons/turrets/bunkers/crawlers/etc
You should use static defenses sparingly and they should by no means make up the bulk of your defense. You should be building the appropriate units to counter your enemies units and defend that way.
By all means make a few at your choke and around your mineral lines, but only enough to provide detection and deterrence to your enemy so that one banshee/dt does not dismantle your base
You should also be scouting your enemy regularly to see what buildings and units he has, and scout the rest of the map regularly to see if any new expansions have been built or if any proxy buildings have been built.
If you go whole hog with static defense you are putting yourself at a massive disadvantage.
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I played a ZvP game last night on Scrap Station, I sent my Ovie in at the beginning and saw a Starport go down, so I built a couple of extra queens, and teched to lair. Knowing a SP build is expensive I sent all my lings into his base to find the whole front blocked off by cannons. Ok I thought. Well he is either going to do drops, proxy, pylons or mass air If I move my lings back to the watch tower I'll know if anything is coming round on foot, so I can mass hydras back at the base and wait for the VR Push.
The VR push came (I supply locked myself a few times so I didn't have the unit count I wanted) and with a bit of difficulty I pushed it back, but I was in good shape, I had lost most of my queens, a bunch of hydras and a few drones. I rebuilt, expanded, and scouted. I noticed he had a base (with cannons) on the island, and he was starting to expand to one of the bases behind the rocks. No problem. I send 3 hydras to the rocks base, to try and get a response whilst expanding to the gold and the base near the watch tower.
I kept building hydras and got to the point where I was wipeing out his entire VR push each time, whilst building more drones, more hydras, mutas and a bunch of corruptors. Then I saw the mothership (which in my view is an all in unit) I sent my hydras and corruptors and a few overseers to deal with it and the carrier VR army underneath it, which I completely wiped out, but lost my gold expansion in the process. NP I can rebuild that. I then restocked my Corruptor army and started building Broodlords, I was now in the position to attack his base. Just as my BLs popped around the corner came a bunch of Colossi I sent my BLs in for the easy kill (lost another expansion, he had about 6 of them) but again, easy enough to rebuild. I then got my BLs, Corruptors, a bunch of hydras all my lings and the muta army I had been keeping in reserve and attacked his rocks expansion, I think all he could muster up to defend that was a few phoenixes and a VR or 3 which I trashed in seconds and he GGd.
Now I did not play that game particularly well, I basically defended each base with the minimum units I could, and often lost a Hatchery to the push, but I was able to rebuild very quickly, and I was able to adapt my play to combat his. In fact him creating the cannon wall in the first place really forced my hand into going Broodlords in the first place (as the best unit zerg has for cracking static defence). So my entire build was really focused in one direction. If he had not built so many cannons, he could probably have expanded sooner, and been in better shape to build more VRs which would have killed me off (due to my supply locking issues at the beginning of the game), he could have also have made use of ground forces such as zealots which in combination with the VRs would have ruined my day.
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Whenever I play against an opponent that walls off
and goes mass static defense, I have learned the game is mine to lose, not his to win. The easiest way for me to win, is to expand, then tech up to Broodlords, and ensure I have sufficient forces to keep the Broodlords alive. This is naturally a very gas heavy build. (I need 6-8 gas mining at the same time) but thats not an issue as the opponent has bunkered in may have only taken their Natural expansion (if that). When it comes to the final battle they may have managed to build a 200/200 army, but chances are it won't be able to handle my army, regardless of my size. Because BLs massacre ground forces, and what ever I have supporting them can handle ground & Air (so basically either hydras or mutas) and I also have a bunch of corruptors available to tank (and fight) any Viking, BC, Phoenixes, VRs that appear.
I'm yet to see a turtling Zerg player btw