I played competitive SC:BW and WC3 and when thinking about building placement and base design you always take in consideration paths of invasion to block weak points, e.g.: access to your workers gathering resources should be blocked as much as possible; create choke points inside the base to use as killing zones; use choke points to break enemy units' path finding; block off ramps with supply buildings to block vision and place a turret with range on the border of the killing zone on the ramp; keep a turret to defend against air drops or flying units coming for your workers.