I've always made my guards low CR but what really sets them apart is the ability to call more!

I usually add: 

Raise the Alarm! - If he/she is not in a secluded location a guard may use an action to raise the alarm. By shouting or ringing a bell the alarm brings an additional guard every 3 rounds, this effect stacks for every guard who raises the alarm.

This makes them much more deadly as an organisation. As guards should be.

This is awesome! I also use this but I've never written a specific ruling for it. 

Guards are easy as easy as fighting an insect that sucks blood

Don't forget that guards come in all flavours. Expect guards of wealthy people/areas to be better equipped than the basic guard. Farm villages might use bows to fight on the open plain, mining towns could use metal martial weapons and heavier armour like breastplates or chain mail. Palace Guards might use heavy armour and possibly halberds.
Feel free to adjust their stats based on experience, training, and role. Here's a list of suggestions with a range of health to challenge players of different levels.
For instance a royal Captain of the guard might use Elite and Leader mods, putting it at 50-225 health with high stats and 3 attacks. 

All of these stat suggestions refer to changes to the base stats given, not the modifiers.
Level:
Basic: 10-25 Health
Skilled: 15-35 health, +1 STR/DEX, +1 WIS
Expert: 20-50 health, +2 STR/DEX, +2 WIS, +1 CON, Possibly +1 Attack
Elite:  25-75 health, +3 STR/DEX, +2 WIS, +2 CON, +1 Attack 
Role:
Leader: 2-3 x Health, +3 CHA, +2 INT, +1 All Other Stats, +1 Attack
Champion: +2 STR, DEX, CON, +1 Attack 
Sentry/Scout: -1 STR, +2 DEX, +2 WIS, DEX Based Gear
Heavy: + 5-20 Health, +3 STR, -2 DEX, +2 CON, Heavy Armour and Weapon
Finishing Touches:
Add feats if you feel the need. Make elite sentries, give them crossbow expert and heavy crossbows and call them "Dragon Slayers" 
Take expert guards, give them pack tactics and call them "Shield Brothers" Just have fun with it.

This is incredible and perfect for when I DM my group.  They are very "stab first ask questions later" mostly because there's no consequences for their murdering sprees.  Raise the Alarm action will bring a new appreciation for more strategic role playing.

this is a great rule!

cool

I really like the fact that people have taken the time to expand on the town-watch aspect of this NPC stat-block.  It is an excellent opportunity to expand on the character, wealth and martial capacity of a local.

In a more “historically accurate” setting, the basic guard presented here is more consistent with a merchant or noble’s personal security force than the peasant-militia which guard medieval towns and hamlets, but that’s just an opportunity for further explanation and details.

ours ran away at the first sight of a gibbering mouther :(

Holy crap. This is an amazing way to scale them up without just having 1000 of them. 

If you really want to discipline your players, give your guards bows and make them work together. Use things like disengage, flanking, and grappling. 

Love this!

Ever tried giving them firearms? (and proficiency) an antimatter rifle would result with a TPK :)

That is such a great idea, I'm going to modify my guards now to include this!

Calling reinforcements really brings these otherwise throw away NPCs to life. Thanks for the idea.

Nah

Quote from ZK_06 >>

Guards are easy as easy as fighting an insect that sucks blood

 I Disagree, stirges are waaaaaay worse

Quote from ShaydyCharacter >>

Don't forget that guards come in all flavours. Expect guards of wealthy people/areas to be better equipped than the basic guard. Farm villages might use bows to fight on the open plain, mining towns could use metal martial weapons and heavier armour like breastplates or chain mail. Palace Guards might use heavy armour and possibly halberds.
Feel free to adjust their stats based on experience, training, and role. Here's a list of suggestions with a range of health to challenge players of different levels.
For instance a royal Captain of the guard might use Elite and Leader mods, putting it at 50-225 health with high stats and 3 attacks. 

All of these stat suggestions refer to changes to the base stats given, not the modifiers.
Level:
Basic: 10-25 Health
Skilled: 15-35 health, +1 STR/DEX, +1 WIS
Expert: 20-50 health, +2 STR/DEX, +2 WIS, +1 CON, Possibly +1 Attack
Elite:  25-75 health, +3 STR/DEX, +2 WIS, +2 CON, +1 Attack 
Role:
Leader: 2-3 x Health, +3 CHA, +2 INT, +1 All Other Stats, +1 Attack
Champion: +2 STR, DEX, CON, +1 Attack 
Sentry/Scout: -1 STR, +2 DEX, +2 WIS, DEX Based Gear
Heavy: + 5-20 Health, +3 STR, -2 DEX, +2 CON, Heavy Armour and Weapon
Finishing Touches:
Add feats if you feel the need. Make elite sentries, give them crossbow expert and heavy crossbows and call them "Dragon Slayers" 
Take expert guards, give them pack tactics and call them "Shield Brothers" Just have fun with it.

 Awesome. Now someone needs to make a table for the CR of all the different combos.

Quote from Helmut_McQuack >>

Quote from ShaydyCharacter >>

Don't forget that guards come in all flavours. Expect guards of wealthy people/areas to be better equipped than the basic guard. Farm villages might use bows to fight on the open plain, mining towns could use metal martial weapons and heavier armour like breastplates or chain mail. Palace Guards might use heavy armour and possibly halberds.
Feel free to adjust their stats based on experience, training, and role. Here's a list of suggestions with a range of health to challenge players of different levels.
For instance a royal Captain of the guard might use Elite and Leader mods, putting it at 50-225 health with high stats and 3 attacks. 

All of these stat suggestions refer to changes to the base stats given, not the modifiers.
Level:
Basic: 10-25 Health
Skilled: 15-35 health, +1 STR/DEX, +1 WIS
Expert: 20-50 health, +2 STR/DEX, +2 WIS, +1 CON, Possibly +1 Attack
Elite:  25-75 health, +3 STR/DEX, +2 WIS, +2 CON, +1 Attack 
Role:
Leader: 2-3 x Health, +3 CHA, +2 INT, +1 All Other Stats, +1 Attack
Champion: +2 STR, DEX, CON, +1 Attack 
Sentry/Scout: -1 STR, +2 DEX, +2 WIS, DEX Based Gear
Heavy: + 5-20 Health, +3 STR, -2 DEX, +2 CON, Heavy Armour and Weapon
Finishing Touches:
Add feats if you feel the need. Make elite sentries, give them crossbow expert and heavy crossbows and call them "Dragon Slayers" 
Take expert guards, give them pack tactics and call them "Shield Brothers" Just have fun with it.

 Awesome. Now someone needs to make a table for the CR of all the different combos.

• Level
  • Basic:  1/8
  • Skilled:  1/8 - 1/4
  • Expert:  1/4 - 1/2
  • Elite:  1/2 - 1
• Role - Multiply with the CR of Level and Round Accordingly
  
  •  Leader:  2x - 3x
  • Champion:  1.5x - 2x
  • Sentry/Scout:  1.5x
  • Heavy:  1.5x

These can All vary depending on Stats, Skills, and Equipment. But it's a good base to get things started as you can tweak the CR rating as needed when building your encounters for your party. (Don't forget about Action Economy as well when building encounters against your players.)

It took my far too long to realize this, but it occurred to me that random townsfolk will use this statblock, because they are off-duty militia.