Remember when you got your first Mortar when you were Town Hall 3? Well now you will know why is it so important!
TH3 = 1 Mortar Level 1 MAX
TH4 = 1 Mortar Level 2 MAX
TH5 = 1 Mortar Level 3 MAX
TH6 = 2 Mortars Level 4 MAX
TH7 = 3 Mortars Level 5 MAX
TH8 = 3 Mortars Level 6 MAX
TH9 = 3 Mortars Level 7 MAX
TH10 = 3 Mortars Level 8 MAX
Town Hall 3
Mortars are the first slash damage defensive building you unlock at Town Hall 3. A good strategy is to keep it centered and as unreachable as possible because it has a red ring around it, that means that if any troop gets inside that ring your mortar will bot be able to strike it
Recommended Strategy - Keep near Storage's and as unreachable as possible
Town Hall 4
Now you unlock more walls and your mortar can be upgraded to level 2 its best to keep it centered and surrounded by walls and make sure its as unreachable as your town hall [unless your a farmer of course!]
Recommended Strategy - Follow Town Hall 3 guideline
Town Hall 5
Ok now you start to get mad that you don't get another mortar but since you unlock 100 lvl5 walls its best to keep it centered and surrounded by buildings with high health [storage's] and by walls
Recommended Strategy - Surround by storage's and by walls
Town Hall 6
Now you get your 2nd Mortar which means that your centered strategy has to change, one strategy is to keep the mortars near the wizard towers, but another one is off centering both mortars to keep their ranges around your base.
Recommended Strategy - Off Center them or Keep them in range of the Wizard Towers [Look at: http://clashofclansbuilder.com/guides/view/1491 for the Importance of the Wizard Towers]
Town Hall 7
Hello! You get your 3rd and last Mortar which means that you have 3 splash buildings protecting your base, a good strategy is to triangulate your mortars to make them protect each other, another strategy is to place all 3 on a diagonal path to keep ur base fully protected
Recommended Strategy - Triangulation of the 3 mortars
Town Hall 8
Now that you get all your mortars level 6 you can place them far away from each other because since their damage is good enough to protect themselves and cover a larger area of your base. Keep all your mortars protected by another defense to ensure your mortars don't get demolished!
Recommended Strategy - Offset all Mortars to ensure maximum defense
Town Hall 9
Now all your Mortars can be level 7 which make them be a prime candidate for the offset strategy but since Town Hall 9's unlock 2 X-Bows a better idea is to keep your Mortars triangulated and not close to the X-Bows [People like to destroy either the X-Bow or the Mortar First so keeping them apart is a good idea!]But in range of the mortar to defend it!
Recommended Strategy - Triangulation with X-Bow Defense
Town Hall 10
Now you unlock the Inferno Tower, so do the same thing you did with your X-Bow and Mortars, keep them all in range of each other but far apart to ensure maximum defense, A good idea is to offset your mortar all over your base and keep your inferno towers close to your town hall
Recommended Strategy - Offset with X-Bow and Inferno Tower Defense