Re: Alarms... check with your alarm company to see if there is a way to wire the alarm to sound if the wires are cut. A friend figured a way to do this with his alarm unit in the event someone cut the phone lines.
Re: Two Story homes... The top of the stairwell is a fair defensive position, but only if you can see down the stairs far enough and/or have a wall to conceal yourself behind.
An older home layout had the stairs do a 90-degree turn halfway up and the stairs emptied into the end of a long hallway. In essence, once someone came around the corner, they could see the length of the hallway. In such cases, defending from a nearby bedroom/bathroom doorway may be a better option.
An escape plan that isn't too detailed is a good idea. For instance... one parent exits first and helps children out of the house. They then flee to a neighbor nearby selected for screening bushes or their known willingness to respond and help. If separated, children should know who's house to go to and to NEVER return home until police or parents say it's safe.
If you have a stealthy intruder, when police arrive they may need access to the house. Keep spare house key(s) attached to a ring along with a light-stick. Attach a clear plastic "map pouch" to the ring too. Inside the map pouch, put a simple illustration of your home's floorplan. When needed, once police arrive, activate the lightstick and throw the assemblage into the yard or driveway. At night the glowing lightstick is self-identifying. If the situation goes south into a hostage or stand-off situation, they have a map of the house and know where you are by cell phone.
If you are sure there are multiple intruders, use your room to maximum advantage. Barricade the door with furniture, the bed or simply blankets...anything to slow down opening the door. Remember that most walls are simply concealment... if they're beating at the door, your first shot(s) thru the door may score or deter them. If a bedroom wall is along the hallway, shots through the wall into the hallway may also be useful. (Protect yourself from incoming on the same wall!) Of course, be sure you know what is across the hallway too.
If multiple intruders are present and forcing the bedroom door, this is the correct time to be prepared to expend ammo at a high rate. Showing of considerable firepower will deter them from entry. Opening a window may also help draw attention to the noise. Contrary to popular opinion, gunfire inside a home is not very loud from outside if the house is closed up in the winter.
BillCA in CA (Unfortunately)