Heat olive oil in a large sauté pan over high heat. Cook shrimp 3 to 4 minutes, until just pink. Add garlic and lime juice and cook 30 more seconds. Season with salt and pepper. Remove from pan and refrigerate until ready to use.
Fill a medium saucepan with water and bring to a boil. Using a small, sharp knife, make an X on each tomato. Fill a medium bowl with ice water. Immerse tomatoes in boiling water 10 to 15 seconds, just until skin loosens. With a slotted spoon, remove tomatoes and drop into ice water. Peel skin; cut tomatoes in half and discard seeds. Dice into 1/4-inch pieces; set aside. Put avocado in a medium bowl and gently toss with lemon juice.
Loosely pack 1/2 cup avocado into a 3-inch round cookie cutter, top with 1/2 cup tomato, and then add 1/2 cup cucumber, gently pressing each layer with a spoon. Carefully remove cutter. Repeat on each plate, and refrigerate until ready to use. (Alternate method: Gently mix salad ingredients and mound on plates.)
Drizzle each tower with dressing, and add 2 shrimp. Garnish with cilantro sprigs.