For the truly paranoid assassin, always wear a disguise. Change faces after each job. A ring of alter self, or a hat of disguise can really pay off for any criminal. A new disguise is essential after every meeting with anyone who can accuse you or create problems with your identity.
To be truly evil, the assassin can use the disguise to imitate a secondary target. Thus, WANTING to be seen to commit the murder or "job" he has to do, and then letting the authorities deal with the "criminal" in that case. Yes, even assassins can be thrifty with their time.
Never use your own front door. But, even better than that is to not have a permanent residence at all. Keep moving. Take on a new identity as needed to keep anybody off your trail. If you look, smell, and walk like a peasant, then nobody will want to bother you. Be boring. Use a good cover story. Even better, have a network of thieves (the guild) set up in positions that can authenticate your cover story. For all intents and purposes, you have become the D&D translation of a CIA "ghost."
If you're a professional assassin, you don't have a home!
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Assuming you do want to guard your residence, caltrops under doors and windows are a good way to slow your enemy...but it also gives them a reason to be suspicious if an ordinary person comes inside unexpectedly.
The roof being the typical perch of thieves in most towns, consider having your roof protected with a grease spell, or slick it up with a mundane, but effectively slippery equivalent. Pig fat? Mind you, it would stink somewhat.
If you have a chimney, a spiked steel grate halfway up might be good. Casting an illusion to have it disappear is even better. Be sure it has a door built into the grate, so you can open it if you need to climb out your chimney as an escape route. If you can affod it, cast Vanish on a small bell, and tie it to the trapdoor with a string.
Escape routes are key. But you only need them if you screwed up! I would have at least one well hidden and unobvious route. A chimney, or underground passage is ideal. An escape route that you can block off is even better, like a loose timber underground, or a lock you can quickly throw on the steel grate as you climb out. Of course, this is assuming that teleport, invisibility, fly, and other such spells are a bit out of reach, or are compromised.
A pet animal with scent would be a good investment. Their scent ability can reveal things you just won't normally catch. Good boy. But, a pet can compromise your cover, especially if you are spotted as an assassin with a dog. Still, a good enough cover identity will usually protect you even from someone who can identify the dog. The disguised assassin can say he picked up a stray, or perhaps disguise the dog as well with magic.
If you have wine or ale, be sure to keep a small stock of sleep poison handy for "special" guests you might invite for a meal. It's good for those nosey neighbors that are getting to close, and is one way to tie up any loose ends.
One-way windows are a good investment as well. Magically enhanced windows that appear closed and shuttered from the outside, can be a good way to see who's at the front door before you invite them in.
Hope this helps. Still need more?