Covering the offensive spawn exits on stage 1, defensive engineers can place sentry guns behind either the hut on the left or the rocks on the right, forcing the attackers to move into a vulnerable position if they are to get a clear shot. Careful of demomen however.
Defending the first control point of stage 1, sentry guns can be placed on the ramp to the second floor or just around the corner, in sight of the control point but not the building entrance, remaining unseen until it is too late for the attackers.
For control point 2 of stage 1, good sentry guns cover the corridor to the control point as well as the control point itself. The most common place is just around the corner from the silos, but I also like the boxes just next to the control point, able to shoot attackers coming down the stairs.
Also forward bases in the upper room on the attackers’ left can be held by either team with a well-placed sentry gun watching the entrance with a bit of cover from the beams.
On stage 2, the attackers’ spawn exit can be covered by sentry guns placed around a variety of corners, but the one on the roof always seems to be the most effective. Watch out for spies coming up the ramp however.
To defend the first point itself, a sentry gun inside the building looking out the door is very dangerous and often missed by and deadly to attackers attacking the point from any route.
When pushed back to defending control point 2, a sentry gun in the corner between the tunnel exits is able to cover both while staying out of sight of enemies coming out the steel door on the left.
The best position for defending the control point itself is also the most common – the sentry gun spam in the corner at the top of the ramp from defensive spawn. 2 or 3 guns here is virtually unassailable without and übercharge assuming pressure is kept on the offensive team.
On the first control point of the third and final stage, sentry guns around both corners of the path leading to the control point have good coverage of the point while being away from enemy fire until the attackers move into a vulnerable position.
Another good location to cover the first control point is in the building with the windows. However make sure the top of the gun isn’t visible to attackers through the windows or it will be ended swiftly by soldiers and demomen.
When defending the final control point of the map, the canal can be defended by sentry guns peeking around corners as support while defensive pressure is applied by defenders (mainly demomen).
To cover both the bridge to the control point and the doorway into the all-important building on the offensive side of the bridge, a sentry gun can be placed at the top of the ramp from the valley below.
Finally, the cover the final control point as well as the bridge, sentry guns can be placed just outside spawn. However make sure they aren’t too vulnerable to fire from enemy soldiers until they get into range (on the bridge).