By 11th level, the tide is beginning to turn for the wizard. He no longer needs to fear for his life, and is beginning to see his hard slog through the low levels having to save up his measly two spells per day (not including cantrips) pay off.
Defensively, nothing can come near Wall of Force. The only problem with Wall of Force is that it does have a negative impact upon your offensive capability. The best bets are Eyebite, or to use Wall of Force as a stopgap until you can have some nice buff spells. Another trick is to have a Wall of Force as big as you can get, and summon some creatures (Summon Monster X) inside the Wall of Force. When you have sufficient beasties, drop the Wall of Force and let them run rampage. Even at only level 11, it's an 11' radius hemisphere, so there should be plenty of room for a few creatures.
Of course, if you want to have decent defense without limiting your offense, Ghostform is always a good bet (Tome & Blood, 5th level spell). It makes you immune to nonmagical weapons, a 50% chance to ignore damage from a corporeal source, prevents you from being grappled, and if things get hairy you can always vanish into the grounds.
NB Ghostform and hemispherical Wall of Force are a nice combination, as you can float through the ground, blast some spells and then withdraw again- particularly impressive if you are also hasted.
For a more conventional artillery approach, Fly sets up a nice platform. However, two added touches are Protection from Arrows (which can be made permanent) and Otiluke's Dispelling Sphere (T&B, 4th level). The latter will (hopefully!) dispel magical arrows coming your way, whilst the former soaks up the now non-magical arrows. Against flying monks and other menaces, the Dispelling Sphere will knock out their fly leaving them to tumble to the ground.
By 11th level, any wizard looking at long-term survival is also glancing at Contingency. Though at 11th you can only tune 3rd level spells to it, Blink, Haste, Displacement and Fly are all good spells to be able to get off automatically. By 12th level, you can tune a Dimension Door to withdraw from combat if things get tricky. By 15th level, Ghostform, Teleport and Wall of Force all become available, meaning that you will almost never be caught off-guard.
Wizardly defense can also be hugely supplemented by the Persistent Spell feat (T&B, FRCS). Though it does bump up the spell level by four, a Persistent Shield can work wonders, amongst others. The best bets are short-duration high-impact and low-level spells, such as Shield. At later levels, Persistent Blink, Persistent Blur and Persistent Displacement are also very nice all-day protections.
High-level wizards get a lion's share of defensive options. I hope that my ramble will be of some use!