I have to say, for a player who is at Level 17, your base is pretty strong! The placements are really good, and there is hardly anything that I would change. There are some points that would be good to change, but its just minor stuff that would be super useful for defence. You dont have to do these, as they are just suggestions
1.) Core Location
The core is in a good spot, but you may wish to move it away from your storage's. This may seem like a good idea, but when the Kracken gets used, they can slice to your network and get access to your core and currency. You may want to move your core more twords the front, and have your scanner able to detect anyone who trys to stealth hack you.
2.) Sentry-Code Gate Positioning
Although it may seem like a good idea, you will want to get your sentries separated. Have one in the other side, so that you still can spread the anti-virus to other nodes if the first is down. Also, if your code gate has 2 sentries attached to it, there is 8-program slots that can destroy it. But, good job on placing the higher leveled one up front.
So, it may seem like a good idea to have the netConnection directly on your scanner. But, for more effective stealth detection. If you move it further away (with the exeption of the connections still being in range of the scanner), you can detect people much more quickly and avoid a stealth attack (but it's highly unlikely due to the codegates filter).
So, feel free to try these out, I find that they have greatly benefited my network, and have had little sucessful attacks from people.