We had a Columba specialist is Circle/Ring spells. She had some interesting ones for people who wandered inside her lab.
A common favourite IMS are:
1.) Circle/Ring versions of Call to Slumber (walk in and off you drop. Wait a day to question, when the parma has worn off.)
2.) A magic item that uses a pressure trigger to end a ReTe effect holding up a large rock that looks like a cieling....
3.) A wall of solid rock with no entrance. Only a specific password opens a MuTe effect to enter the lab. Circulate a fake password. In fact, it opens at any password, but closes when someone is halfway through, crushing them (since its a return to normal, no MR). Only the correct password lets one enter safely.
4.) Permanent Circles of Wards against demons, faeries and curious scullion. Cheap, easy, fast.
5.) A wooden floor when you enter the lab, which is intentionally creaky. There is an PeHe effect triggered to a InHe effect that destroys the wood when it creeks. Down buddy falls into a specially prepared pit.
6.) A series of dangerous animals which are reseased from PeIm enchanted cages when someone breaks a ring on the floor. They are natural animals and are angry.
7.) A PeAn effect with Circle Range/ RING duration. Destroying the ring on the floor surrounds the intruder with a dozen wolves (Harmless Magic). This is an often overlooked application of hermetic magic: a non-human anything can be temporarily destroyed, especially with Harmless Magic, and sprung back into existence. This can likewise be done with anything: Fire, rocks, acid, swords, whatever.
The best defenses are indirect damage effects. They all rely on the secrecy of the sanctum. DO NOT SHOW OFF YOUR DEFENSES. The chief benefit a magus has is that an enemy does not know what to expect when he/she walks into a sanctum. Circle effects are great, because they can be created without vis and without spending seasons to enchant. This makes them cheaper and more flexible, as circle effects can be substituted or changed over time.
Don't rely too much on penetrating effects. Whatever your power, there's still a more powerful opponent.
Don't neglect the danger of mundanes. Demons et al. of course always use mundane agents.