Exactly. Three pages to get the most basic solution; design your bases for defense.
Aliens only come in through the entrance, large hanger, and the basic radar tower. Rebuild your radar to advanced later to get rid of one more spot, though the radar tower can be an excellent sniper spot. Early on I try to get a couple guys there fast to start picking them off at the entrance, because radar is often near the entrance. Your troops often spawn in your living quarters, so build 2 close to the entrance. Use the entrance as your choke point. This means you lose a few spots of area to build in, but it's worth it for the ease of defense, and you can always build more bases. Most of the time, they come in through the entrance. I haven't had any spawn inside the command center or anywhere inside the base.
Once the attack starts, get your guys to the entrance choke point and stake out all corridors coming from it, if designed right, will not be more than 2. Deal with any aliens as they come down, try to entice them if you can. While you deal with them, send 3 or 4 of your guys on ahead to try to take the radar tower or large hanger so you can flank the ones at the entrance. If you've designed your base right, there won't be very many corridors and a 2x2 squad arrangement for them will get you there without leaving any place unsearched.
Base defense is simple if you plan for it. Should take no longer than any other mission.
EDITED: Troop spawn points.