Locks are pretty easy to defeat with force on most doors, and can be bypassed by going through a window. After decent locks I'd be looking at inexpensive hardening (lights on outside, lights on timers inside, thorny/spiky plants underneath windows, eliminating cover where appropriate), then an alarm, and after that more expensive hardening (reinforced doors, security film on windows, high-end locks).

I have a large dog with a terrifying bark. The only time I don't view him as being better than an alarm system is when we're on vacation and he's not home to protect my property.

When we're not home, it's been reported, he gives the mailman a half-hearted bark or two. When I'm home he barks rather more. When my wife is home and I am not, he goes to full-on Eat The Mailman Mode. This is a good thing.